From 970874094f9eec76cb522a8294cab6bdd3b44ef4 Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Tue, 9 Jun 2026 10:59:07 -0700
Subject: [PATCH 1/6] Address WHATWG #12150 feedback and add HTMLButtonBehavior
 alternative

Apply feedback from WHATWG #12150 and from multiple internal review
rounds across the main explainer's User-facing problem, User research,
Dynamic behaviors, Alt API design 3, and HTMLSubmitButtonBehavior
Properties sections. Add Alt API design 5 (standard-defined strings)
covering the shapes raised in WHATWG #12150. Move Behavior composition
and conflict resolution into Future work. Replace prescriptive
attachInternals() lifecycle guidance with neutral language. Cut
duplicate examples and convert the Behavior lifecycle table to prose.

Add htmlbuttonbehavior-with-type-attribute.md, a side document
exploring an alternative direction that collapses the three button
modes (submit, reset, generic-button) into a single HTMLButtonBehavior
whose type property mirrors native <button>'s type attribute, and
declares behaviors via a static class property (symmetric with
static formAssociated) with platform-owned instantiation. Authors
look up the platform-created instance through
internals.behaviors.get(HTMLButtonBehavior).

Net contribution vs. upstream/main: 2 files, +358 / -236.
---
 PlatformProvidedBehaviors/explainer.md        | 333 +++++-------------
 .../htmlbuttonbehavior-with-type-attribute.md | 261 ++++++++++++++
 2 files changed, 358 insertions(+), 236 deletions(-)
 create mode 100644 PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md

diff --git a/PlatformProvidedBehaviors/explainer.md b/PlatformProvidedBehaviors/explainer.md
index 42d738fa..6346b791 100644
--- a/PlatformProvidedBehaviors/explainer.md
+++ b/PlatformProvidedBehaviors/explainer.md
@@ -10,14 +10,14 @@
 
 ## Introduction
 
-Custom element authors frequently need their elements to use platform behaviors that are currently exclusive to native HTML elements, such as [form submission](https://github.com/WICG/webcomponents/issues/814), [popover invocation](https://github.com/whatwg/html/issues/9110), [label behaviors](https://github.com/whatwg/html/issues/5423#issuecomment-1517653183), [form semantics](https://github.com/whatwg/html/issues/10220), and [radio button grouping](https://github.com/whatwg/html/issues/11061#issuecomment-3250415103). The motivation of this proposal is to give custom element authors visibility into the same protocols, lifecycle hooks, and internal state that native elements use. Platform-provided behaviors name the platform-internal logic HTML already runs for native elements and expose it for reuse, rather than asking authors to reimplement that logic in Javascript.
+Custom element authors frequently need their elements to use platform behaviors that are currently exclusive to native HTML elements, such as [form submission](https://github.com/WICG/webcomponents/issues/814), [popover invocation](https://github.com/whatwg/html/issues/9110), [label behaviors](https://github.com/whatwg/html/issues/5423#issuecomment-1517653183), [form semantics](https://github.com/whatwg/html/issues/10220), and [radio button grouping](https://github.com/whatwg/html/issues/11061#issuecomment-3250415103). The motivation of this proposal is to give custom element authors visibility into the same protocols, lifecycle hooks, and internal state that native elements use. Platform-provided behaviors name the platform-internal logic HTML already runs for native elements and expose it for reuse, rather than asking authors to reimplement that logic in Javascript. This proposal starts with form submission because there is no API today to make a custom element participate in implicit submission (`form.requestSubmit()` only handles explicit activation), and `<button type="submit">` is a well-understood single-element conformance target that exemplifies every part of the framework the proposal needs to define (focusability, implicit role, pseudo-class participation, default activation, form integration) and helps validate the framework.
 
 ## User-facing problem
 
 Custom element authors can't access native behaviors that are built into native HTML elements. This forces them to either:
 
 1. Use customized built-ins (`is/extends` syntax), which have [Shadow DOM limitations](https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#elements_you_can_attach_a_shadow_to) and [can't use the ElementInternals API](https://github.com/whatwg/html/issues/5166).
-2. Try to reimplement native logic in JavaScript, which is error-prone, often less performant, and cannot replicate certain platform-internal behaviors.
+2. Try to reimplement native logic in JavaScript, which is error-prone, often less performant, and cannot replicate certain platform-internal behaviors (notably the form-control pseudo-class set such as `:default` and the native focusability algorithm).
 3. Accept that their custom elements simply can't do what native elements can do.
 
 This creates a gap between what's possible with native elements and custom elements, limiting web components and forcing developers into suboptimal patterns.
@@ -48,6 +48,8 @@ This proposal is informed by:
 
 3. Real-world use cases from frameworks that work around these limitations:
 
+   Frameworks rebuild `<button>` rather than use the native one for two main reasons. First, style encapsulation: a design system wants its button to be styled and slotted entirely within the component's shadow DOM. Second, internal structure: design systems frequently ship complex button variants like split buttons, buttons with internal grids, or buttons that surface a secondary action, none of which the native element accommodates. The cost is that form submission, implicit submission, focusability, the implicit role, and pseudo-classes all have to be reimplemented or partially worked around.
+
    - [Shoelace](https://github.com/shoelace-style/shoelace/blob/next/src/components/button/button.component.ts#L180): Uses `ElementInternals` but still requires manual wiring to intercept the click event on its internal shadow button (as shown below) and can't support implicit submission ("Enter to submit").
 
    ```javascript
@@ -73,11 +75,7 @@ This proposal is informed by:
 
 These frameworks mirror a native `<button>` by having one custom element class whose form activation is selected by the `type` attribute. The same pattern appears across other design systems including [Adobe Spectrum Web Components](https://github.com/adobe/spectrum-web-components/blob/main/1st-gen/packages/button/src/ButtonBase.ts) (`<sp-button>` with `type` attribute, hidden `<button type="submit">` proxy) and [Microsoft Fluent UI Web Components](https://github.com/microsoft/fluentui/blob/master/packages/web-components/src/button/button.base.ts) (`<fluent-button>` with `type` attribute, built on the FAST web component foundation).
 
-### Why start with form submission?
-
-1. There's a clear gap with implicit form submission.
-2. Form submission has clear semantics, making it useful for validating the overall pattern (lifecycle, conflict resolution, accessibility integration) before expanding to more complex behaviors.
-3. There's no API to make a custom element participate in implicit form submission as `form.requestSubmit()` only handles explicit activation.
+Most of these gaps have userland workarounds. `ElementInternals.role` for an ARIA role, `tabindex` to make the host focusable, and form-associated custom elements already participate in `:disabled` and validity pseudo-classes through `ElementInternals`. However, there is no way to make `:default` match the current default submit button because that selector is tied to native form-control semantics. This proposal closes that gap and the rest of the form-submission integration that today only `<button type="submit">` provides.
 
 ## Proposed approach
 
@@ -95,7 +93,9 @@ this._submitBehavior.formAction = '/custom';
 const submitBehavior = this._internals.behaviors.find(
   b => b instanceof HTMLSubmitButtonBehavior
 );
-submitBehavior?.disabled = true;
+if (submitBehavior) {
+  submitBehavior.disabled = true;
+}
 ```
 
 Each behavior names the specific platform logic it engages:
@@ -130,9 +130,9 @@ This proposal introduces `HTMLSubmitButtonBehavior`, which mirrors the submissio
 
 ### Behaviors are not opaque tokens
 
-A recurring concern about consolidating form-control semantics into an opt-in is that web authors will not be able to figure out what a given opt-in actually does. This proposal addresses that concern directly:
+A recurring concern about consolidating form-control semantics into an opt-in is that web authors will not be able to figure out what a given opt-in actually does. This proposal addresses that concern:
 
-- Each element behavior maps to a single native pattern (e.g., `HTMLSubmitButtonBehavior` provides exactly the semantics of `<button type="submit">`). Future element behaviors will need to follow the same naming convention, each pinned to a single native pattern.
+- Each element behavior maps to a single native pattern (e.g., `HTMLSubmitButtonBehavior` provides exactly the semantics of `<button type="submit">`). Future element behaviors will need to follow the same naming convention.
 - Each behavior is specified in terms of existing HTML algorithms. The behavior is the union of those algorithms, applied to a custom element.
 - Web authors can override individual defaults. This already works today for role: `internals.role` overrides a behavior's default role without replacing the behavior. The same layering pattern can extend to other defaults (focusability, keyboard activation, and similar) if and when future proposals add the corresponding primitives on `ElementInternals`. The [layering example in Alternative 7](#alternative-7-low-level-primitives-on-elementinternals) walks through what that would look like.
 
@@ -150,83 +150,28 @@ Each behavior exposes properties and methods from its corresponding native eleme
 - `formTarget`
 - `labels` - read-only, delegates to `ElementInternals.labels`
 - `name`
+- `title` - surfaced through the user agent's default tooltip UI
 - `value`
 
-To expose properties like `disabled` or `formAction` to external code, authors define getters and setters that delegate to the behavior.
-
-```javascript
-class CustomSubmitButton extends HTMLElement {
-  constructor() {
-    super();
-    this._submitBehavior = new HTMLSubmitButtonBehavior();
-    this._internals = this.attachInternals({ behaviors: [this._submitBehavior] });
-  }
-
-  get disabled() {
-    return this._submitBehavior.disabled;
-  }
-
-  set disabled(val) {
-    this._submitBehavior.disabled = val;
-  }
-
-  get formAction() {
-    return this._submitBehavior.formAction;
-  }
-
-  set formAction(val) {
-    this._submitBehavior.formAction = val;
-  }
-}
-```
-
-Authors are responsible for attribute reflection. If the author wants HTML attributes on their custom element to affect the `behavior`, they need to observe and forward those attributes using `attributeChangedCallback`:
-
-```javascript
-class CustomButton extends HTMLElement {
-  static observedAttributes = ['disabled', 'formaction'];
-
-  attributeChangedCallback(name, oldValue, newValue) {
-    if (name === 'disabled') {
-      this._submitBehavior.disabled = newValue !== null;
-    } else if (name === 'formaction') {
-      this._submitBehavior.formAction = newValue ?? '';
-    }
-  }
-}
-```
+To expose these properties to external code, authors define getters and setters on the host that delegate to the behavior. See [Use case: Design system button](#use-case-design-system-button) for a complete worked example.
 
-Form override values (`formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget`, `name`, `value`) are read from `behavior` properties. Setting a value declaratively in markup (e.g., `<my-button formaction="/save">`) or programmatically (e.g. `setAttribute('formaction', '/save')`) has no effect on form submission unless the author explicitly forwards that attribute to the `behavior`. However, some element attributes are applied to form submission as part of the existing [form-associated custom element](https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements) mechanism, independently of behaviors:
-
-| Element attribute | Applied via form-associated custom element mechanics | Notes |
-|-------------------|------------------------------------------------------|-------|
-| `form` | Yes | Standard form association by ID. `behavior.form` is read-only and delegates to `ElementInternals.form`, which reflects this association. |
-| `disabled` | Yes | Standard form control disabling. Combined with `behavior.disabled`. |
-| `name`, `value` | No | Only `behavior.name` and `behavior.value` are read on submission. |
-| `formaction`, `formenctype`, `formmethod`, `formtarget` | No | Only `behavior` properties are read. |
+Form override values (`formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget`, `name`, `value`) are read from `behavior` properties. Setting a value declaratively in markup (e.g., `<my-button formaction="/save">`) or programmatically (e.g. `setAttribute('formaction', '/save')`) has no effect on form submission unless the author explicitly forwards that attribute to the `behavior`. However, some element attributes are applied to form submission as part of the existing [form-associated custom element](https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements) mechanism.
 
 ### Behavior lifecycle
 
-When `attachInternals()` is called with behaviors, each behavior is attached to the element:
-
-| Event | Effect |
-|-------|--------|
-| `attachInternals()` called with behaviors | Each behavior is attached. Event handlers become active. Default ARIA role is applied unless overridden by `ElementInternals.role`. |
-| Element disconnected from DOM | Behavior state is preserved. Event handlers remain conceptually attached but inactive. |
-| Element reconnected to DOM | Event handlers become active again. Behavior state (e.g., `formAction`, `disabled`) is preserved. |
+Calling `attachInternals()` with behaviors adds each behavior's event handlers and the default ARIA role is applied. Disconnecting the element from the DOM leaves event handlers attached but suspends them, while behavior state (`formAction`, `disabled`, etc.) is preserved. Reconnecting reactivates event handling against the preserved state.
 
 ### Dynamic behaviors
 
-Behaviors are immutable after `attachInternals()`. Adding, removing, or replacing behaviors after attachment is not supported in this proposal (e.g., a `<custom-button>` that gains submit behavior when an attribute appears and loses it when the attribute is removed). Dynamically adding or removing behaviors the following questions:
+Three parts are identified on "dynamism of behaviors":
 
-- What happens to form ownership if the behavior is removed mid-life?
-- What does the element's role become, and is the role transition observable to assistive technologies?
-- How does focusability need to transition while adding and removing behaviors?
-- What if activation is in flight (mousedown happened with the behavior attached, mouseup arrives after the behavior is removed)?
+- Set: which behaviors are attached
+- State: the configurable properties on the behavior instance
+- Participation: whether the behavior's contribution is currently active
 
-`<input>` can be cited as a platform precedent for adding and removing capabilities and properties mid-life. However, `<input>` exposes its entire IDL surface at all times and the `type` attribute changes which subset of properties is currently active; it doesn't add or remove anything from the element. The same is true for `<a>`, where setting or unsetting `href` changes how the element behaves but doesn't change what's exposed on it.
+This proposal commits to making the set static after `attachInternals()` to allow for attribute values to participate in choosing which behaviors a custom element should have, makes the state mutable for the life of the behavior, and proposes a path to get participation togglability in [Alternative API design 3: Behavior-scoped behavior.disabled](#alternative-api-design-3-behavior-scoped-behaviordisabled). Adding, removing, or replacing behaviors after attachment is not supported.
 
-For elements that want to express toggle-like behavior at runtime, the proposed path is captured in [Alternative API design 3: Behavior-scoped behavior.disabled](#alternative-api-design-3-behavior-scoped-behaviordisabled).
+This mirrors how native HTML elements change capability at runtime. `<button>` exposes the same IDL surface whether `type` is `"submit"`, `"reset"`, or `"button"`. The `type` attribute selects which subset of that surface is currently meaningful, but it never changes what the element exposes. The same is true for `<a>`, where setting or unsetting `href` changes how the element behaves but doesn't grow or shrink its IDL surface. Treating behaviors as a fixed set after attachment keeps custom elements consistent with that model.
 
 ### Duplicate behaviors
 
@@ -264,118 +209,12 @@ Behaviors are instantiated with `new` and passed to `attachInternals()`:
 
 - `behaviors` option in `attachInternals({ behaviors: [...] })` accepts behavior instances.
 - `behaviors` property on `ElementInternals` is a read-only `FrozenArray`.
-- Developers hold direct references to their behavior instances.
 
 *Note: An ordered array is preferred over a set because order may be significant for [conflict resolution](#behavior-composition-and-conflict-resolution). `behaviors` uses a `FrozenArray` because behaviors are immutable after attachment.*
 
-Developers hold direct references to their behavior instances. This follows the [W3C design principle that classes should have constructors](https://www.w3.org/TR/design-principles/#constructors) that allow authors to create and configure instances, and it extends naturally to future developer-defined behaviors that follow the same `new` + attach pattern.
-
-*For future developer-defined behaviors:*
-
-```javascript
-class TooltipBehavior {
-  #content = '';
-
-  behaviorAttachedCallback(internals) { /* ... */ }
-
-  get content() { return this.#content; }
-  set content(val) { this.#content = val; }
-}
-
-// In custom element constructor:
-this._tooltipBehavior = new TooltipBehavior();
-this._internals = this.attachInternals({ behaviors: [this._tooltipBehavior] });
-
-// Access state directly.
-this._tooltipBehavior.content = 'Helpful tooltip text';
-```
-
-The choice of which behaviors to attach can depend on attributes at element initialization. Web authors may use `connectedCallback` for this because by the time the element is connected, its attributes are observable, including for elements created via `document.createElement()` and then configured with `setAttribute()` before insertion.
-
-```javascript
-class MyButton extends HTMLElement {
-  static formAssociated = true;
-  #internals;
-  #submitBehavior;
-
-  connectedCallback() {
-    // Defer to connectedCallback because parser-set attributes
-    // aren't present yet in the constructor.
-    // connectedCallback also works for elements created with
-    // document.createElement() and configured with
-    // setAttribute() before insertion.
-    if (this.#internals) {
-      return;
-    }
-    const behaviors = [];
-    if (this.hasAttribute('submit')) {
-      this.#submitBehavior = new HTMLSubmitButtonBehavior();
-      behaviors.push(this.#submitBehavior);
-    }
-    // If behavior selection doesn't depend on attributes,
-    // attachInternals() can run in the constructor instead.
-    this.#internals = this.attachInternals({ behaviors });
-  }
-}
-```
-
-### Behavior composition and conflict resolution
-
-For the built-in behaviors currently under consideration and mentioned in this document (`HTMLSubmitButtonBehavior`, `HTMLButtonBehavior`, `HTMLResetButtonBehavior`, etc.), there probably isn't a practical use case for combining them. However, attaching multiple behaviors is not prohibited. If multiple behaviors are provided, conflicts are resolved using order of precedence: the position of behaviors in the array determines which behavior's value takes effect.
-
-Consider the following example of behaviors that could potentially be compatible, `HTMLLabelBehavior` and `HTMLSubmitButtonBehavior`:
-
-```javascript
-class LabeledSubmitButton extends HTMLElement {
-  static formAssociated = true;
-
-  constructor() {
-    super();
-    this._labelBehavior = new HTMLLabelBehavior();
-    this._submitBehavior = new HTMLSubmitButtonBehavior();
-    this._internals = this.attachInternals({
-      behaviors: [this._labelBehavior, this._submitBehavior]
-    });
-
-    this.addEventListener('click', (event) => {
-      console.log('Author click handler');
-    });
-  }
-}
-customElements.define('labeled-submit-button', LabeledSubmitButton);
-
-const btn = document.createElement('labeled-submit-button');
-
-// For properties where an element can only have one value (ARIA role, `disabled`,
-// `formAction`, etc.), last-in-wins applies:
-
-// The element's implicit role is "button" (from HTMLSubmitButtonBehavior, last in list).
-console.log(btn.computedRole);  // "button"
-
-// If the author sets `internals.role`, that takes precedence over all behavior defaults.
-this._internals.role = 'link';
-console.log(btn.computedRole);  // "link"
-
-// However, the element is disabled if any behavior's `disabled` is `true`. When
-// disabled, the entire element is affected: all behavior default actions are blocked,
-// the element is removed from tab order, and it matches `:disabled`.
-submitBehavior.disabled = true;
-labelBehavior.disabled = false;
-console.log(btn.matches(':disabled'));  // true
-```
-
-For events, behavior responses follow the platform's existing default action model:
-
-1. The event dispatches through the DOM.
-2. All author-registered event listeners run during dispatch.
-3. After dispatch completes, each behavior's default action runs unless a listener called `preventDefault()`.
+Developers hold direct references to their behavior instances. This follows the [W3C design principle that classes should have constructors](https://www.w3.org/TR/design-principles/#constructors) that allow authors to create and configure instances, and extends naturally to author-defined behavior classes explored in the separate [developer-defined behaviors](developer-defined-behaviors.md) document.
 
-When `btn` is clicked:
-- Author event listeners run during event dispatch.
-- `HTMLSubmitButtonBehavior`'s default action runs (form submits).
-- `HTMLLabelBehavior`'s default action runs (delegates focus to associated control).
-- If any listener called `preventDefault()`, both form submission and focus delegation are cancelled
-- `stopImmediatePropagation()` prevents subsequent listeners on the same element from running, but does not affect behavior default actions.
+`attachInternals()` can be called from the constructor, `connectedCallback`, or `attributeChangedCallback`. The constructor works for parser-created elements (the parser sets attributes before construction), as the [Use case: Design system button](#use-case-design-system-button) example demonstrates. For workflows where attributes are set after construction (for example, the `document.createElement()` + `setAttribute()` workflow), deferring attachment to `connectedCallback` or `attributeChangedCallback` lets the element see those attributes when selecting behaviors.
 
 ### Feature detection
 
@@ -397,12 +236,7 @@ if (typeof HTMLSubmitButtonBehavior !== 'undefined') {
 
 ### Other considerations
 
-This proposal supports common web component patterns:
-
-- A child class extends the parent's functionality and retains access to the `ElementInternals` object and its active behaviors.
-- This proposal targets autonomous custom elements that need platform behaviors (e.g., when needing Shadow DOM and custom APIs or building a design system component that is an autonomous custom element). Making native elements more flexible (Customizable Select, open-stylable controls) is valuable and complementary, but doesn't completely eliminate the need for autonomous custom elements.
-- Platform-provided behaviors are JavaScript-dependent, as is any autonomous custom element. If script fails to load, the element receives no behavior—this is true with or without this proposal.
-- Custom elements using behaviors can still follow progressive enhancement patterns: use `<slot>` to render fallback content, provide `<noscript>` alternatives, and design markup to be readable without JavaScript.
+- Custom elements using behaviors can follow progressive enhancement patterns: use `<slot>` to render fallback content, provide `<noscript>` alternatives, and design markup to be readable without JavaScript. If script fails to load, the element receives no behavior, which is true for any autonomous custom element with or without this proposal.
 - Because behaviors are pinned to existing algorithms, this framework also enables polyfilling: authors can approximate new behaviors in *userland* before native support ships (see [Developer-defined behaviors](#developer-defined-behaviors) in [Future Work](#future-work)).
 - While this proposal uses an imperative API, the design supports future declarative custom elements. Once a declarative syntax for `ElementInternals` is established, attaching behaviors could be modeled as an attribute, decoupling behavior from the JavaScript class definition. The following snippet shows a hypothetical example:
 
@@ -415,9 +249,7 @@ This proposal supports common web component patterns:
 
 ### Use case: Design system button
 
-While this proposal only introduces `HTMLSubmitButtonBehavior`, the example below references `HTMLResetButtonBehavior` and `HTMLButtonBehavior` to illustrate how switching would work once additional behaviors become available in the future.
-
-A design system can use delayed `attachInternals()` to determine the behavior based on the initial `type` attribute. This approach uses a single class while keeping behaviors immutable after attachment.
+While this proposal only introduces `HTMLSubmitButtonBehavior`, the example below references `HTMLResetButtonBehavior` and `HTMLButtonBehavior` to illustrate how a custom button could leverage this framework once additional behaviors become available in the future.
 
 ```javascript
 class DesignSystemButton extends HTMLElement {
@@ -430,7 +262,7 @@ class DesignSystemButton extends HTMLElement {
 
   constructor() {
     super();
-    this.attachShadow({ mode: "open" });
+    this.attachShadow({ mode: 'open' });
   }
 
   connectedCallback() {
@@ -659,53 +491,81 @@ These behaviors are *technically* compatible because:
 
 Even with an explicit role, this pattern may confuse users who expect consistent behavior from elements announced as buttons or links. Authors should evaluate whether their use case requires both behaviors or if a single semantic (button or link) would better serve users.
 
-#### Conflicting behaviors
+## Future work
+
+The behavior pattern can be extended to additional behaviors. The following are concrete near-term candidates:
+
+- **Generic Buttons**: `HTMLButtonBehavior` for non-submitting buttons (popover invocation, commands).
+- **Reset Buttons**: `HTMLResetButtonBehavior` for form resetting.
+- **Labels**: `HTMLLabelBehavior` for `for` attribute association and focus delegation.
+- **Radio Groups**: `HTMLRadioGroupBehavior` for `name`-based mutual exclusion.
+
+### Behavior composition and conflict resolution
+
+The current proposal only introduces `HTMLSubmitButtonBehavior`, so composition with other behaviors is a forward-looking concern. Once additional behaviors land (see candidates above), an element may attach more than one. Behaviors can conflict in several ways
+
+- Different default roles.
+- Different ways to handle the same event.
+- The same property with different values
 
-Some behaviors are inherently mutually exclusive.
+The proposed resolution policy is last-in-wins for properties and additive for event default actions. The position of behaviors in the array determines which behavior's value takes effect.
 
 ```javascript
-this._checkboxBehavior = new HTMLCheckboxBehavior();
-this._radioBehavior = new HTMLRadioGroupBehavior();
-this.attachInternals({ 
-  behaviors: [this._checkboxBehavior, this._radioBehavior]
-});
-```
+class MultiActionButton extends HTMLElement {
+  static formAssociated = true;
 
-| Capability | HTMLCheckboxBehavior | HTMLRadioGroupBehavior |
-|------------|---------------------|------------------------|
-| `checked` property | Toggles independently on/off | Checking one unchecks others from the group |
-| Click handling | Toggles `checked` state | Sets `checked = true` (radios don't toggle off) |
-| ARIA role | `role="checkbox"` | `role="radio"` |
-| `aria-checked` | `"true"` / `"false"` / `"mixed"` | `"true"` / `"false"` |
+  constructor() {
+    super();
+    this._submitBehavior = new HTMLSubmitButtonBehavior();
+    this._resetBehavior = new HTMLResetButtonBehavior();
+    this._internals = this.attachInternals({
+      behaviors: [this._submitBehavior, this._resetBehavior]
+    });
 
-The result is incoherent: the element has radio semantics for the `checked` property (group coordination) but the checkbox's click handler might still try to toggle off, or vice versa depending on event handler ordering (if applying "last-in-wins"). An element cannot meaningfully be both a checkbox and a radio button.
+    this.addEventListener('click', (event) => {
+      console.log('Author click handler');
+    });
+  }
+}
+customElements.define('multi-action-button', MultiActionButton);
 
-## Future work
+const form = document.querySelector('form');
+const btn = document.createElement('multi-action-button');
+form.append(btn);
 
-The behavior pattern can be extended to additional behaviors:
+// `disabled` is defined by both behaviors. Last-in-wins applies, so the host's
+// effective disabled state matches HTMLResetButtonBehavior (last in the array).
+btn._submitBehavior.disabled = true;
+btn._resetBehavior.disabled = false;
+console.log(btn.matches(':disabled'));  // false
 
-- **Generic Buttons**: `HTMLButtonBehavior` for non-submitting buttons (popover invocation, commands).
-- **Reset Buttons**: `HTMLResetButtonBehavior` for form resetting.
-- **Inputs**: `HTMLInputBehavior` for text entry, validation, and selection APIs.
-- **Labels**: `HTMLLabelBehavior` for `for` attribute association and focus delegation.
-- **Forms**: `HTMLFormBehavior` for custom elements acting as form containers.
-- **Radio Groups**: `HTMLRadioGroupBehavior` for `name`-based mutual exclusion.
-- **Tables**: `HTMLTableBehavior` for table layout semantics and accessibility.
+// `formAction` is only defined by HTMLSubmitButtonBehavior, so the platform
+// reads it directly from that behavior.
+btn._submitBehavior.formAction = '/submit';
 
-Future behaviors would also manage their own relevant pseudo-classes:
+// Role resolution: both behaviors expose role "button". An author-set
+// `internals.role` still overrides any behavior default.
+console.log(btn.computedRole);  // "button"
+btn._internals.role = 'link';
+console.log(btn.computedRole);  // "link"
+```
 
-| Behavior | CSS Pseudo-classes |
-|----------|--------------------|
-| `HTMLCheckboxBehavior` | `:checked`, `:indeterminate` |
-| `HTMLInputBehavior` | `:valid`, `:invalid`, `:required`, `:optional`, `:placeholder-shown` |
-| `HTMLRadioGroupBehavior` | `:checked` |
-| `HTMLResetButtonBehavior` | `:default` (if only reset button in form) |
+For events, behavior responses follow the platform's existing default action model:
 
-### Developer-defined behaviors
+1. The event dispatches through the DOM.
+2. All author-registered event listeners run during dispatch.
+3. After dispatch completes, each behavior's default action runs unless a listener called `preventDefault()`.
 
-A future extension of this proposal could allow developers to define their own reusable behaviors by subclassing an `ElementBehavior` base class. This would enable patterns like custom tooltip behaviors, polyfilling upcoming platform behaviors, and composing developer-defined behaviors with platform-provided ones.
+When `btn` is clicked:
+- Author event listeners run during event dispatch.
+- `HTMLSubmitButtonBehavior`'s default action runs (the form submits).
+- `HTMLResetButtonBehavior`'s default action runs (the form resets).
+- If any listener called `preventDefault()`, both default actions are cancelled.
+- `stopImmediatePropagation()` prevents subsequent listeners on the same element from running, but does not affect behavior default actions.
+
+### Developer-defined behaviors
 
-This direction is explored in a separate document: [Developer-defined behaviors](developer-defined-behaviors.md). It is not part of the current proposal and should be treated as forward-looking exploration.
+A future extension of this proposal could allow developers to define their own reusable behaviors by subclassing an `ElementBehavior` base class. This would enable patterns like polyfilling upcoming platform behaviors and composing developer-defined behaviors with platform-provided ones. This direction is explored in a separate document: [Developer-defined behaviors](developer-defined-behaviors.md). It is not part of the current proposal and should be treated as forward-looking exploration.
 
 ### Behaviors in native HTML elements
 
@@ -870,7 +730,7 @@ While valuable, this can be a parallel effort. Even if all native elements were
 
 ### Alternative 7: Low-level primitives on ElementInternals
 
-Expose individual primitives directly on `ElementInternals`. Authors compose every native capability piece by piece.
+Expose individual primitives on `ElementInternals`. Authors compose every native capability piece by piece.
 
 **Pros:**
 - Maximum flexibility: authors compose exactly what they need.
@@ -1039,7 +899,9 @@ class CustomSubmitButton extends HTMLElement {
 
 ### Alternative API design 3: Behavior-scoped `behavior.disabled`
 
-For elements that want to freely turn behaviors on and off at runtime, the proposed path is to attach every relevant behavior at `attachInternals()` and use a per-behavior off switch to turn each behavior's capabilities on or off from the outside. The behaviors stay attached and their IDL surfaces stay exposed on the host; only their participation toggles. Under the current design, setting `behavior.disabled = true` disables the host element. An alternative semantics is for `behavior.disabled` to disable only that behavior's contribution. The behavior remains attached (the proposal still forbids removing it after `attachInternals()`), its attribute and IDL surface stays exposed on the host, but the behavior becomes inert: its activation handler is not invoked, it does not contribute to focusability, and its implicit ARIA role and pseudo-class participation are suppressed. Element-level disabled state continues to drive the host's actual disabled state.
+For elements that want to turn behaviors on and off at runtime, the proposed path is to attach every relevant behavior at `attachInternals()` and to use a per-behavior `disabled` property to flip each behavior between participating and inert. The behaviors stay attached and their IDL surfaces stay exposed on the host; only their participation toggles.
+
+Under the current main proposal, setting `behavior.disabled = true` disables the host element. The alternative semantics described here is that `behavior.disabled` only disables that behavior's contribution. The behavior remains attached (the proposal still forbids removing it after `attachInternals()`) and its attribute and IDL surface stay exposed on the host, but the behavior becomes inert: its activation handler is not invoked, it does not contribute to focusability, and its implicit ARIA role and pseudo-class participation are suppressed. The host's overall disabled state is driven independently by the [form-associated custom element mechanism](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled). A behavior whose `disabled` is `false` still does not activate if the host itself is disabled.
 
 ```javascript
 class CustomButton extends HTMLElement {
@@ -1068,17 +930,16 @@ class CustomButton extends HTMLElement {
 }
 ```
 
-**Pros:**
+Toggling `behavior.disabled` reuses the same recomputation paths the platform already runs for form ownership, implicit role, focusability, and pseudo-class invalidation on native and custom elements. Changing `behavior.disabled` between events of a single interaction (for example, between `mousedown` and `mouseup`) queues the change. The change applies at end-of-interaction, between event tasks. This mirrors how the platform already handles `disabled` mutations made during click dispatch on native elements: the activation that started against the enabled behavior completes, and the next activation sees the new state.
 
-- Dynamic de-application of a behavior object stays disallowed, the on/off switch lives on the behavior itself.
-- Answers the "we need dynamic behaviors" use cases without reintroducing add/remove: A `<custom-button>` whose `type` attribute selects between submit and reset, a custom toggle that swaps between switch and press-button shape, a component that activates different behaviors on mobile versus desktop are all addressed by attaching every behavior up front and toggling each one's `disabled` in response to attribute or state changes.
+**Pros:**
+- Dynamic de-application of a behavior object stays disallowed. The on/off toggle lives on the behavior itself.
+- Answers the "we need dynamic behaviors" use cases without introducing add/remove. A `<custom-button>` whose `type` attribute selects between submit and reset, a custom toggle that swaps between switch and press-button shape, and a component that activates different behaviors on mobile versus desktop are all addressed by attaching every behavior up front and toggling each one's `disabled` in response to attribute or state changes.
 - The capabilities of each attached behavior stay discoverable on the host regardless of disabled state.
+- The host-disabled vs behavior-disabled separation lets authors model both axes independently.
 
 **Cons:**
-
-- There is still some complexity to resolve as this would likely affect how conflict resolution works and even if `disabled` is the right attribute to use or if we should introduce a special attribute or event that signifies behavior disablement.
-- Mid-interaction state changes remain ambiguous. Changing `behavior.disabled` between events of a single interaction (for example, between `mousedown` and `mouseup`, or between `keydown` and `keyup` on a key activation) raises the same question as runtime add/remove: does the activation that started against the enabled behavior still complete, or does it become inert at the moment the toggle happens?
-- The current `behavior.disabled` semantics matches what authors most often expect from a `disabled` flag set on an element.
+- Reusing `disabled` matches what authors expect from a `disabled` flag, but it overloads a term that already has element-scoped meaning under the form-associated custom element mechanism.
 
 If developer feedback agrees that multi-behavior on/off is needed, this is the proposed path.
 
diff --git a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
new file mode 100644
index 00000000..7a029772
--- /dev/null
+++ b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
@@ -0,0 +1,261 @@
+# Alternative: HTMLButtonBehavior with internal type attribute
+
+This document explores an alternative direction for [platform-provided behaviors](explainer.md). This alternative differs in two ways:
+
+1. It collapses the three button modes into a single `HTMLButtonBehavior` whose `type` property mirrors the `type` attribute of native `<button>`. The behavior's `type` toggles which button mode the host participates in. Everything else (focusability, role, pseudo-classes, the activation path) is constant across types.
+2. It declares behaviors via a `static behaviors` class property, letting the platform own behavior instantiation. Authors access the platform-created instances through a new `behaviors` collection on `ElementInternals`.
+
+Four things motivate this alternative:
+
+1. Modeling the button types as three separate behaviors duplicates most of the surface to express activation differences.
+2. Web authors who want a single custom element class that can switch between submit, reset, and generic-button at runtime want one element with one mutable property. A single `HTMLButtonBehavior` with a mutable `type` avoids having to define what happens when two button-shaped behaviors are attached at once and one is toggled off.
+3. It matches native `<button>` and frameworks also use this shape.
+4. The platform owns instantiation; authors look up state through `ElementInternals`.
+
+The composition rules in this alternative are the same as the main proposal: multiple behaviors of different shapes can be attached together, and behavior composition and conflict resolution defines how they interact.
+
+## Proposed approach
+
+A platform-provided behavior is a set of methods, values, and platform-protocol hooks that a custom element can participate in, which today are reserved to native HTML elements. This alternative introduces a `behaviors` static class property that declares which behaviors the platform attaches to instances of the class.
+
+The platform reads `static behaviors` at `customElements.define()` time and stores it on the custom element definition, the same way it reads `static formAssociated`. When an element is created (`new`, parser upgrade, or `customElements.upgrade()`), the platform instantiates one of each declared behavior per host and associates them with the element. This mirrors how a form-associated custom element participates in form submission and form-related lifecycle callbacks.
+
+`attachInternals()` is the author's access route to those already-existing instances, exposed through a new `behaviors` collection on `ElementInternals`. The set of behaviors attached to a host is fixed at the class declaration; the behavior instances themselves remain mutable post-attach.
+
+```javascript
+class CustomButton extends HTMLElement {
+  static formAssociated = true;
+  // Attach behavior.
+  static behaviors = [HTMLButtonBehavior];
+
+  #internals;
+  constructor() {
+    super();
+    // Access the behavior state directly.
+    const buttonBehavior = this.#internals.behaviors.get(HTMLButtonBehavior);
+
+    // Modify the behavior's properties.
+    buttonBehavior.formAction = '/custom';
+    buttonBehavior.type = 'reset';
+  }
+}
+```
+
+Each behavior names the specific platform logic it engages:
+
+- Event handling and activation
+- ARIA defaults (implicit role and ARIA properties)
+- Focusability
+- CSS pseudo-classes
+- Configurable data and state owned by the behavior
+- Platform protocol hooks
+
+### HTMLButtonBehavior
+
+This alternative introduces `HTMLButtonBehavior`, which mirrors native `<button>`. The behavior has a `type` property (`'submit'`, `'reset'`, or `'button'`, defaulting to `'submit'`) that selects the active button mode. The `type` is mutable for the life of the behavior.
+
+- User activation (click, Enter, Space, implicit submission) reaches the behavior through the same DOM event-dispatch path as native elements, including `stopPropagation` and `preventDefault`. The behavior's activation handler is invoked after default-prevention checks the same way `<button>`'s is.
+- The behavior provides a default implicit `role="button"`. Authors can override the role through `internals.role`.
+- The custom element participates in sequential focus navigation using the same focusability logic native elements use, with `tabindex` and disabled state following the established rules.
+- The same logic that toggles `:default`, `:disabled`/`:enabled`, `:focus`, and `:focus-visible` on native elements applies to the behavior's host. The `:default` pseudo-class only matches when `type === 'submit'` and the host is the form's default submit button, matching native behavior.
+- Mirrored `HTMLButtonElement` properties are available on the behavior instance. They are configurable per-element and mutable for the life of the behavior.
+- Form ownership applies whenever `type` is `'submit'` or `'reset'`. Activation behavior depends on `type`: `'submit'` triggers form submission and implicit submission; `'reset'` triggers form reset; `'button'` does neither. The wiring matches the corresponding native `<button type="...">` activation behavior.
+
+What the `type` property gates:
+
+| `type` value | Activation behavior | `:default` match | Implicit submission | Form ownership |
+|--------------|--------------------|------------------|--------------------|----------------|
+| `'submit'` (default) | Submits the form | Yes, if default submit button | Yes | Yes |
+| `'reset'` | Resets the form | No | No | Yes |
+| `'button'` | None | No | No | No |
+
+`HTMLButtonBehavior` builds on top of [form-associated custom elements (FACEs)](https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements). The custom element still has to opt in to form association with `static formAssociated = true` for submission to actually fire when `type` is `'submit'` or `'reset'`. Without it, `behavior.form` is always `null` and activation is a no-op even when the element is inside a form. This is a divergence from native `<button>`, which submits its form without any explicit opt-in.
+
+| Scenario | Behavior |
+|----------|----------|
+| Form-associated element inside a form, `type === 'submit'` | Activation triggers submission, participates in implicit submission, matches `:default`. Invoker attributes on the host (`commandfor`, `popovertarget`) are ignored, matching native `<button type="submit">`. |
+| Form-associated element inside a form, `type === 'reset'` | Activation triggers form reset. Invoker attributes on the host are ignored, matching native `<button type="reset">`. |
+| Form-associated element inside a form, `type === 'button'` | Activation is a no-op for form behavior. Invoker attributes on the host (`commandfor`, `popovertarget`) fire as usual. |
+| Form-associated element outside a form, or non-form-associated element | `behavior.form` is `null`. With no form owner, the host behaves like a native `<button>` without a form owner for all `type` values: there is no form to submit or reset, but invoker attributes on the host (`commandfor`, `popovertarget`) fire as usual and the element still gets `role="button"` and implicit focusability. |
+
+### Behaviors are not opaque tokens
+
+A recurring concern about consolidating form-control semantics into an opt-in is that web authors will not be able to figure out what a given opt-in actually does. This proposal addresses that concern:
+
+- Each element behavior maps to a single native pattern (e.g., `HTMLButtonBehavior` provides exactly the semantics of `<button>`). Future element behaviors will need to follow the same naming convention.
+- Each behavior is specified in terms of existing HTML algorithms. The behavior is the union of those algorithms, applied to a custom element.
+- Web authors can override individual defaults. This already works today for role: `internals.role` overrides a behavior's default role without replacing the behavior. The same layering pattern can extend to other defaults (focusability, keyboard activation, and similar) if and when future proposals add the corresponding primitives on `ElementInternals`. The [layering example in Alternative 7](#explainer.md#alternative-7-low-level-primitives-on-elementinternals) walks through what that would look like.
+
+### Accessing behavior state
+
+Each behavior exposes properties and methods from its corresponding native element. Behaviors can be accessed via `internals.behaviors`. For `HTMLButtonBehavior`, the following properties are available (mirroring [`HTMLButtonElement`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLButtonElement)):
+
+**Properties:**
+- `type` - selects the active button mode (`'submit'`, `'reset'`, or `'button'`); defaults to `'submit'`.
+- `disabled` - The element is effectively disabled if either `behavior.disabled` is `true` or the element is disabled via attribute or is a descendant of `<fieldset disabled>` ([spec](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled)).
+- `form` - read-only, delegates to `ElementInternals.form`. Form ownership only affects activation when `type` is `'submit'` or `'reset'`.
+- `name`, `value` - submitter name and value. Read on submission (`type === 'submit'`).
+- `formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget` - submission overrides. Read on submission (`type === 'submit'`).
+- `labels` - read-only, delegates to `ElementInternals.labels`.
+- `title` - surfaced through the user agent's default tooltip UI.
+
+To expose these properties to external code, authors define getters and setters on the host that delegate to the behavior. See [Use case: Design system button](#use-case-design-system-button) for a complete worked example.
+
+### Behavior lifecycle
+
+| Event | What happens |
+|-------|--------------|
+| Element creation (`new`, parser upgrade, or `customElements.upgrade()`) | The platform instantiates each declared behavior with its defined defaults. The behavior's `type` selects the initial active button mode (`'submit'` by default). Role, focusability, and pseudo-class participation are active from this point. |
+| `attachInternals()` | `internals.behaviors` is populated with the already-existing behavior instances. Authors now have read/write access through `internals.behaviors.get(ElementBehavior)`. |
+| Host connected | Form association runs if `formAssociated = true`. The behavior's `form` is resolved. |
+| `behavior.type` set to a new value | Form ownership, role, focusability, and pseudo-class state are recomputed. See [Dynamic behaviors](#dynamic-behaviors). |
+| Host disconnected | Form association detaches. The behavior remains attached for when the host re-connects. |
+
+### Dynamic behaviors
+
+Setting `behavior.type` to a new value toggles the activation path and which pseudo-classes match.
+
+```javascript
+const buttonBehavior = this.#internals.behaviors.get(HTMLButtonBehavior);
+buttonBehavior.type = 'reset'; // Was 'submit', now 'reset'.
+```
+
+Each subsystem affected by `type` is recomputed through the same paths the platform already runs for native `<button>` when its `type` attribute changes.
+
+| Subsystem | Recompute when `behavior.type` is set |
+|-----------|--------------------------------------|
+| Form ownership | Re-association runs. `type === 'button'` detaches the behavior from form ownership; `type === 'submit'` or `'reset'` attaches it. |
+| Activation behavior | The next activation runs against the new `type`. |
+| `:default` match | Re-evaluated. Only matches when `type === 'submit'` and the host is the form's default submit button. |
+| Implicit ARIA role | Unchanged. The role is `"button"` for all `type` values, so no recompute is needed. |
+| Focusability | Unchanged. Focusability is the same for all `type` values. |
+
+Setting `behavior.type` to an unknown string does not throw. The behavior coerces the value to the default state (`'submit'`), and the getter returns the canonical keyword for the active state. This matches the [Auto state](https://html.spec.whatwg.org/multipage/form-elements.html#attr-button-type-auto-state) that `<button>`'s `type` content attribute uses as the missing-value and invalid-value default.
+
+Changing `behavior.type` between events of a single interaction (for example, between `mousedown` and `mouseup`, or between `keydown` and `keyup` on a key activation) queues the change. The change applies at end-of-interaction, between event tasks. This mirrors how the platform already handles `type` mutations on a native `<button>` during click dispatch.
+
+### Duplicate behaviors
+
+Listing the same behavior class twice in `static behaviors` (for example, `static behaviors = [HTMLButtonBehavior, HTMLButtonBehavior]`) throws a `TypeError` at `customElements.define()` time. A host has at most one instance of each behavior class.
+
+### API design
+
+Authors declare behaviors via a static class property; the platform creates and exposes the instances:
+
+- `static behaviors` is an array of behavior classes (not instances).
+- The platform instantiates each declared behavior with its defined defaults at element creation.
+- `ElementInternals.behaviors` is a read-only, Map-like collection keyed by behavior class. `internals.behaviors.get(HTMLButtonBehavior)` returns the instance for this host. Entries can't be added or removed at runtime.
+- Web authors hold references to behavior instances by looking them up through `internals.behaviors` once and caching the reference.
+
+### Feature detection
+
+Web authors can detect whether behaviors are supported by checking for the existence of behavior classes on the global scope:
+
+```javascript
+if (typeof HTMLButtonBehavior !== 'undefined') {
+  // Behaviors are supported.
+  class MyButton extends HTMLElement {
+    static formAssociated = true;
+    static behaviors = [HTMLButtonBehavior];
+
+    #internals;
+    constructor() {
+      super();
+      this.#internals = this.attachInternals();
+    }
+  }
+  customElements.define('my-button', MyButton);
+} else {
+  // Fall back to manual event handling.
+  class MyButton extends HTMLElement {
+    static formAssociated = true;
+
+    #internals;
+    constructor() {
+      super();
+      this.#internals = this.attachInternals();
+      this.addEventListener('click', () => {
+        this.#internals.form?.requestSubmit(this);
+      });
+    }
+  }
+  customElements.define('my-button', MyButton);
+}
+```
+
+### Other considerations
+
+This proposal supports common web component patterns:
+
+- Custom elements using behaviors can follow progressive enhancement patterns: use `<slot>` to render fallback content, provide `<noscript>` alternatives, and design markup to be readable without JavaScript. If script fails to load, the element receives no behavior, which is true for any autonomous custom element with or without this proposal.
+- Because behaviors are pinned to existing algorithms, this framework also enables polyfilling: authors can approximate new behaviors in *userland* before native support ships (see [Developer-defined behaviors](#developer-defined-behaviors) in [Future Work](#future-work)).
+- While this proposal uses an imperative API, the design supports future declarative custom elements.
+
+### Use case: Design system button
+
+A design system can use a single class with one `HTMLButtonBehavior` and forward the host's `type` attribute to the behavior.
+
+```javascript
+class DesignSystemButton extends HTMLElement {
+  static formAssociated = true;
+  static behaviors = [HTMLButtonBehavior];
+  static observedAttributes = ['type', 'formaction'];
+
+  #internals;
+  #buttonBehavior;
+
+  constructor() {
+    super();
+    this.#internals = this.attachInternals();
+    this.#buttonBehavior = this.#internals.behaviors.get(HTMLButtonBehavior);
+    this.attachShadow({ mode: 'open' });
+  }
+
+  connectedCallback() {
+    this.shadowRoot.innerHTML = '<slot></slot>';
+  }
+
+  attributeChangedCallback(name, _oldValue, newValue) {
+    switch (name) {
+      case 'type': {
+        if (newValue === 'submit' || newValue === 'reset' || newValue === 'button') {
+          this.#buttonBehavior.type = newValue;
+        }
+        break;
+      }
+      case 'formaction': {
+        this.#buttonBehavior.formAction = newValue ?? '';
+        break;
+      }
+    }
+  }
+}
+
+customElements.define('ds-button', DesignSystemButton);
+```
+
+```html
+<form action="/save">
+  <ds-button type="submit">Save</ds-button>
+  <ds-button type="reset">Reset</ds-button>
+  <ds-button type="button" onclick="openHelp()">Help</ds-button>
+</form>
+```
+
+Setting the `type` attribute at runtime flips the active mode through `attributeChangedCallback`, the way an author would expect:
+
+```javascript
+document.querySelector('ds-button').setAttribute('type', 'reset');
+// The behavior's type is now 'reset'. The next activation triggers form reset.
+```
+
+## Comparison with current [explainer](explainer.md)
+
+| Question | Main proposal | This alternative |
+|----------|---------------|------------------|
+| How is the behavior attached? | `attachInternals({ behaviors: [ new HTMLSubmitButtonBehavior, ...] })`. | `static behaviors = [HTMLButtonBehavior]`. |
+| How are submit, reset, and generic-button modes modeled? | `HTMLSubmitButtonBehavior`, `HTMLResetButtonBehavior`, `HTMLButtonBehavior`. | `HTMLButtonBehavior` with a `type` property. |
+| How does the author access the behavior? | Through the cached instance reference. | `internals.behaviors.get(HTMLButtonBehavior)`. |
+| How do authors switch modes at runtime? | See [Use case: Design system button](explainer.md#use-case-design-system-button) and [Alternative API design 3](explainer.md#alternative-api-design-3-behavior-scoped-behaviordisabled). | Set `behavior.type` to the new value. |
+| Conflict between two button-shaped behaviors attached together | Defined by array-order / last-in-wins among participating behaviors. | A host can only have one `HTMLButtonBehavior`, last-in-wins for the rest of the participating behaviors. |
+| Mapping to native `<button>` | Three behaviors mapping to the three `type` values. | One-to-one with `HTMLButtonElement`, including the `type` attribute. |

From 82fcd431c19602a0bc25deaa6b61a7b8e6c475e1 Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Tue, 9 Jun 2026 13:58:10 -0700
Subject: [PATCH 2/6] Drop title from behavior properties, note
 HTMLElement-inherited surface

`HTMLButtonElement`'s IDL adds form-control properties on top of
`HTMLElement` but does not override `title`, `tabIndex`, `hidden`,
`lang`, `dir`, `accessKey`, or `draggable`. All of these are pure
`HTMLElement` surface that custom elements already inherit. Including
`title` in the behavior interface would be redundant and would imply
the behavior is involved in tooltip surfacing when it is not.

Drop `title` from the Properties lists in both the main explainer
(`HTMLSubmitButtonBehavior`) and the type-attribute alternative
(`HTMLButtonBehavior`). Add a short note after each list pointing
out that `HTMLElement`-inherited properties work directly on the
host without behavior involvement.
---
 PlatformProvidedBehaviors/explainer.md                         | 3 ++-
 .../htmlbuttonbehavior-with-type-attribute.md                  | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/PlatformProvidedBehaviors/explainer.md b/PlatformProvidedBehaviors/explainer.md
index 6346b791..1ed5991d 100644
--- a/PlatformProvidedBehaviors/explainer.md
+++ b/PlatformProvidedBehaviors/explainer.md
@@ -150,9 +150,10 @@ Each behavior exposes properties and methods from its corresponding native eleme
 - `formTarget`
 - `labels` - read-only, delegates to `ElementInternals.labels`
 - `name`
-- `title` - surfaced through the user agent's default tooltip UI
 - `value`
 
+*Note: `HTMLButtonElement` adds the properties listed above on top of `HTMLElement`. Custom elements already inherit the global `HTMLElement` IDL surface (`title`, `tabIndex`, `hidden`, etc.). Web authors can use these properties on the host as they would on any element.*
+
 To expose these properties to external code, authors define getters and setters on the host that delegate to the behavior. See [Use case: Design system button](#use-case-design-system-button) for a complete worked example.
 
 Form override values (`formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget`, `name`, `value`) are read from `behavior` properties. Setting a value declaratively in markup (e.g., `<my-button formaction="/save">`) or programmatically (e.g. `setAttribute('formaction', '/save')`) has no effect on form submission unless the author explicitly forwards that attribute to the `behavior`. However, some element attributes are applied to form submission as part of the existing [form-associated custom element](https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements) mechanism.
diff --git a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
index 7a029772..0f7adbf1 100644
--- a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
+++ b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
@@ -97,7 +97,8 @@ Each behavior exposes properties and methods from its corresponding native eleme
 - `name`, `value` - submitter name and value. Read on submission (`type === 'submit'`).
 - `formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget` - submission overrides. Read on submission (`type === 'submit'`).
 - `labels` - read-only, delegates to `ElementInternals.labels`.
-- `title` - surfaced through the user agent's default tooltip UI.
+
+*Note: `HTMLButtonElement` adds the properties listed above on top of `HTMLElement`. Custom elements already inherit the global `HTMLElement` IDL surface (`title`, `tabIndex`, `hidden`, etc.). Web authors can use these properties on the host as they would on any element.*
 
 To expose these properties to external code, authors define getters and setters on the host that delegate to the behavior. See [Use case: Design system button](#use-case-design-system-button) for a complete worked example.
 

From dd3ffe8aa9744c2bab438d6fd7eae291c8a67cb5 Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Mon, 29 Jun 2026 13:57:28 -0700
Subject: [PATCH 3/6] Refine HTMLButtonBehavior type-attribute alternative

---
 .../htmlbuttonbehavior-with-type-attribute.md | 38 ++++++++++---------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
index 0f7adbf1..87a0677c 100644
--- a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
+++ b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
@@ -25,12 +25,15 @@ The platform reads `static behaviors` at `customElements.define()` time and stor
 ```javascript
 class CustomButton extends HTMLElement {
   static formAssociated = true;
-  // Attach behavior.
+  // Declare which behaviors the platform attaches to each instance.
   static behaviors = [HTMLButtonBehavior];
 
   #internals;
   constructor() {
     super();
+    // attachInternals() is the access route to the platform-created behaviors.
+    this.#internals = this.attachInternals();
+
     // Access the behavior state directly.
     const buttonBehavior = this.#internals.behaviors.get(HTMLButtonBehavior);
 
@@ -41,6 +44,8 @@ class CustomButton extends HTMLElement {
 }
 ```
 
+In this model authors never construct behaviors. `new HTMLButtonBehavior()` throws an `"Illegal constructor"` `TypeError`, the same as other platform-owned objects such as `ElementInternals`, because a free-standing instance would have no host to act on. The instance is obtained only through `internals.behaviors.get(HTMLButtonBehavior)` after `attachInternals()`.
+
 Each behavior names the specific platform logic it engages:
 
 - Event handling and activation
@@ -52,22 +57,15 @@ Each behavior names the specific platform logic it engages:
 
 ### HTMLButtonBehavior
 
-This alternative introduces `HTMLButtonBehavior`, which mirrors native `<button>`. The behavior has a `type` property (`'submit'`, `'reset'`, or `'button'`, defaulting to `'submit'`) that selects the active button mode. The `type` is mutable for the life of the behavior.
+This alternative introduces `HTMLButtonBehavior`, which mirrors native `<button>`.
 
-- User activation (click, Enter, Space, implicit submission) reaches the behavior through the same DOM event-dispatch path as native elements, including `stopPropagation` and `preventDefault`. The behavior's activation handler is invoked after default-prevention checks the same way `<button>`'s is.
+- The behavior has a `type` property (`'submit'` (default), `'reset'`, or `'button'`) that selects the active button mode. The `type` is mutable for the life of the behavior.
+- User activation (click, Enter, Space, implicit submission) reaches the behavior through the same DOM event-dispatch path as native elements.
 - The behavior provides a default implicit `role="button"`. Authors can override the role through `internals.role`.
-- The custom element participates in sequential focus navigation using the same focusability logic native elements use, with `tabindex` and disabled state following the established rules.
-- The same logic that toggles `:default`, `:disabled`/`:enabled`, `:focus`, and `:focus-visible` on native elements applies to the behavior's host. The `:default` pseudo-class only matches when `type === 'submit'` and the host is the form's default submit button, matching native behavior.
+- The custom element with HTMLButtonBehavior participates in sequential focus navigation, with `tabindex` and disabled state following established rules.
+- The same logic that toggles `:default`, `:disabled`/`:enabled`, `:focus`, and `:focus-visible` on native elements applies to the behavior's host. The `:default` pseudo-class only matches when `type === 'submit'` and the host is the form's default submit button.
 - Mirrored `HTMLButtonElement` properties are available on the behavior instance. They are configurable per-element and mutable for the life of the behavior.
-- Form ownership applies whenever `type` is `'submit'` or `'reset'`. Activation behavior depends on `type`: `'submit'` triggers form submission and implicit submission; `'reset'` triggers form reset; `'button'` does neither. The wiring matches the corresponding native `<button type="...">` activation behavior.
-
-What the `type` property gates:
-
-| `type` value | Activation behavior | `:default` match | Implicit submission | Form ownership |
-|--------------|--------------------|------------------|--------------------|----------------|
-| `'submit'` (default) | Submits the form | Yes, if default submit button | Yes | Yes |
-| `'reset'` | Resets the form | No | No | Yes |
-| `'button'` | None | No | No | No |
+- Form ownership applies whenever `type` is `'submit'` or `'reset'`. Activation behavior depends on `type`: `'submit'` triggers form submission and implicit submission; `'reset'` triggers form reset; `'button'` does generic activation.
 
 `HTMLButtonBehavior` builds on top of [form-associated custom elements (FACEs)](https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements). The custom element still has to opt in to form association with `static formAssociated = true` for submission to actually fire when `type` is `'submit'` or `'reset'`. Without it, `behavior.form` is always `null` and activation is a no-op even when the element is inside a form. This is a divergence from native `<button>`, which submits its form without any explicit opt-in.
 
@@ -109,10 +107,10 @@ To expose these properties to external code, authors define getters and setters
 | Element creation (`new`, parser upgrade, or `customElements.upgrade()`) | The platform instantiates each declared behavior with its defined defaults. The behavior's `type` selects the initial active button mode (`'submit'` by default). Role, focusability, and pseudo-class participation are active from this point. |
 | `attachInternals()` | `internals.behaviors` is populated with the already-existing behavior instances. Authors now have read/write access through `internals.behaviors.get(ElementBehavior)`. |
 | Host connected | Form association runs if `formAssociated = true`. The behavior's `form` is resolved. |
-| `behavior.type` set to a new value | Form ownership, role, focusability, and pseudo-class state are recomputed. See [Dynamic behaviors](#dynamic-behaviors). |
+| `behavior.type` set to a new value | Form ownership, role, focusability, and pseudo-class state are recomputed. See [Mutating the `type` property](#mutating-the-type-property). |
 | Host disconnected | Form association detaches. The behavior remains attached for when the host re-connects. |
 
-### Dynamic behaviors
+### Mutating the `type` property
 
 Setting `behavior.type` to a new value toggles the activation path and which pseudo-classes match.
 
@@ -189,8 +187,12 @@ if (typeof HTMLButtonBehavior !== 'undefined') {
 This proposal supports common web component patterns:
 
 - Custom elements using behaviors can follow progressive enhancement patterns: use `<slot>` to render fallback content, provide `<noscript>` alternatives, and design markup to be readable without JavaScript. If script fails to load, the element receives no behavior, which is true for any autonomous custom element with or without this proposal.
-- Because behaviors are pinned to existing algorithms, this framework also enables polyfilling: authors can approximate new behaviors in *userland* before native support ships (see [Developer-defined behaviors](#developer-defined-behaviors) in [Future Work](#future-work)).
-- While this proposal uses an imperative API, the design supports future declarative custom elements.
+- Because behaviors are pinned to existing algorithms, this framework also enables polyfilling: authors can approximate new behaviors in *userland* before native support ships.
+- While this proposal uses an imperative API, the design supports future declarative custom elements. A declarative form would attach behaviors by a registered string name rather than a class reference. Each built-in behavior would have a canonical token (for example, `HTMLButtonBehavior` registered as `"button"`).
+
+```html
+<my-button behaviors="button">Help</my-button>
+```
 
 ### Use case: Design system button
 

From 9f81c4fd00693d142a9d642d50189f917e83ca54 Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Mon, 29 Jun 2026 14:02:13 -0700
Subject: [PATCH 4/6] Fix broken Alternative 7 cross-link in type-attribute
 alternative

---
 .../htmlbuttonbehavior-with-type-attribute.md                   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
index 87a0677c..028f5495 100644
--- a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
+++ b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
@@ -82,7 +82,7 @@ A recurring concern about consolidating form-control semantics into an opt-in is
 
 - Each element behavior maps to a single native pattern (e.g., `HTMLButtonBehavior` provides exactly the semantics of `<button>`). Future element behaviors will need to follow the same naming convention.
 - Each behavior is specified in terms of existing HTML algorithms. The behavior is the union of those algorithms, applied to a custom element.
-- Web authors can override individual defaults. This already works today for role: `internals.role` overrides a behavior's default role without replacing the behavior. The same layering pattern can extend to other defaults (focusability, keyboard activation, and similar) if and when future proposals add the corresponding primitives on `ElementInternals`. The [layering example in Alternative 7](#explainer.md#alternative-7-low-level-primitives-on-elementinternals) walks through what that would look like.
+- Web authors can override individual defaults. This already works today for role: `internals.role` overrides a behavior's default role without replacing the behavior. The same layering pattern can extend to other defaults (focusability, keyboard activation, and similar) if and when future proposals add the corresponding primitives on `ElementInternals`. The [layering example in Alternative 7](explainer.md#alternative-7-low-level-primitives-on-elementinternals) walks through what that would look like.
 
 ### Accessing behavior state
 

From 838965cfb447e7f9862f279e3d5bb7135690b934 Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Wed, 1 Jul 2026 15:38:46 -0700
Subject: [PATCH 5/6] Address PR #1340 review feedback on behavior lifecycle
 and events

---
 PlatformProvidedBehaviors/explainer.md | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/PlatformProvidedBehaviors/explainer.md b/PlatformProvidedBehaviors/explainer.md
index 1ed5991d..af215b58 100644
--- a/PlatformProvidedBehaviors/explainer.md
+++ b/PlatformProvidedBehaviors/explainer.md
@@ -160,7 +160,7 @@ Form override values (`formAction`, `formEnctype`, `formMethod`, `formNoValidate
 
 ### Behavior lifecycle
 
-Calling `attachInternals()` with behaviors adds each behavior's event handlers and the default ARIA role is applied. Disconnecting the element from the DOM leaves event handlers attached but suspends them, while behavior state (`formAction`, `disabled`, etc.) is preserved. Reconnecting reactivates event handling against the preserved state.
+Calling `attachInternals()` with behaviors adds each behavior's event handlers and applies that behavior's default ARIA (e.g. the implicit `role="button"`). Event handlers remain attached whether or not the element is connected, matching native elements. Behavior state (`formAction`, `disabled`, etc.) is preserved across disconnect and reconnect. Disconnecting only affects behavior that needs the element to be in the document (e.g., a submit behavior cannot submit a form while detached because there is no form to submit to, but it does not stop the behavior's event handlers from running).
 
 ### Dynamic behaviors
 
@@ -551,18 +551,9 @@ btn._internals.role = 'link';
 console.log(btn.computedRole);  // "link"
 ```
 
-For events, behavior responses follow the platform's existing default action model:
+For events, a behavior's default action plugs into the platform's existing [event dispatch](https://dom.spec.whatwg.org/#concept-event-dispatch) and [activation behavior](https://dom.spec.whatwg.org/#eventtarget-activation-behavior) model. This proposal does not change dispatch itself. The only behavior-specific point is that each attached behavior contributes its own default action, and those default actions are additive and gated by `preventDefault()`.
 
-1. The event dispatches through the DOM.
-2. All author-registered event listeners run during dispatch.
-3. After dispatch completes, each behavior's default action runs unless a listener called `preventDefault()`.
-
-When `btn` is clicked:
-- Author event listeners run during event dispatch.
-- `HTMLSubmitButtonBehavior`'s default action runs (the form submits).
-- `HTMLResetButtonBehavior`'s default action runs (the form resets).
-- If any listener called `preventDefault()`, both default actions are cancelled.
-- `stopImmediatePropagation()` prevents subsequent listeners on the same element from running, but does not affect behavior default actions.
+When `btn` is clicked, author listeners run during dispatch, then both `HTMLSubmitButtonBehavior`'s default action (the form submits) and `HTMLResetButtonBehavior`'s (the form resets) run, unless a listener called `preventDefault()`, which cancels both. `stopImmediatePropagation()` still only prevents subsequent listeners on the same element from running; it does not affect behavior default actions.
 
 ### Developer-defined behaviors
 
@@ -1100,6 +1091,7 @@ Many thanks for valuable feedback and advice from:
 - [Alex Russell](https://github.com/slightlyoff)
 - [Andy Luhrs](https://github.com/aluhrs13)
 - [Daniel Clark](https://github.com/dandclark)
+- [Greg Whitworth](https://github.com/gregwhitworth)
 - [Hoch Hochkeppel](https://github.com/mhochk)
 - [Justin Fagnani](https://github.com/justinfagnani)
 - [Keith Cirkel](https://github.com/keithamus)

From 1bfc8ca07a13220b51f84119d36ef0781e919fae Mon Sep 17 00:00:00 2001
From: Ana Sollano Kim <ansollan@microsoft.com>
Date: Wed, 1 Jul 2026 15:39:16 -0700
Subject: [PATCH 6/6] Model behavior composition as an ActivationBehavior
 category hierarchy

---
 .../htmlbuttonbehavior-with-type-attribute.md | 124 +++++++++++++++++-
 1 file changed, 118 insertions(+), 6 deletions(-)

diff --git a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
index 028f5495..b649fee0 100644
--- a/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
+++ b/PlatformProvidedBehaviors/htmlbuttonbehavior-with-type-attribute.md
@@ -55,9 +55,125 @@ Each behavior names the specific platform logic it engages:
 - Configurable data and state owned by the behavior
 - Platform protocol hooks
 
+### Behavior categories and composition
+
+Behaviors are organized into a set of categories, expressed as abstract base classes.
+
+```mermaid
+classDiagram
+    class ElementBehavior
+    class ActivationBehavior {
+        <<abstract>>
+        +activationBehavior(event)
+        +legacyPreActivationBehavior(event)
+        +legacyCanceledActivationBehavior(event)
+    }
+    class EmbeddedContentBehavior {
+        <<abstract>>
+    }
+    class HTMLButtonBehavior
+    class HTMLAnchorBehavior
+    class HTMLLabelBehavior
+    class HTMLRadioGroupBehavior
+    class HTMLCheckboxBehavior
+    class HTMLImageBehavior
+    ElementBehavior <|-- ActivationBehavior
+    ElementBehavior <|-- EmbeddedContentBehavior
+    ActivationBehavior <|-- HTMLButtonBehavior
+    ActivationBehavior <|-- HTMLAnchorBehavior
+    ActivationBehavior <|-- HTMLLabelBehavior
+    ActivationBehavior <|-- HTMLRadioGroupBehavior
+    ActivationBehavior <|-- HTMLCheckboxBehavior
+    EmbeddedContentBehavior <|-- HTMLImageBehavior
+```
+
+`ActivationBehavior` and `EmbeddedContentBehavior` are abstract category bases; the concrete leaves (`HTMLButtonBehavior`, `HTMLAnchorBehavior`, `HTMLLabelBehavior`, `HTMLRadioGroupBehavior`, `HTMLCheckboxBehavior`, `HTMLImageBehavior`) are what an author attaches.
+
+An element attaches at most one behavior per category. Two behaviors in the same category are mutually exclusive, and attaching both throws a `TypeError`:
+
+```javascript
+static behaviors = [HTMLButtonBehavior, HTMLAnchorBehavior]; // throws!
+// TypeError: an element can have at most one ActivationBehavior;
+// HTMLButtonBehavior and HTMLAnchorBehavior cannot be combined.
+```
+
+Behaviors in different categories compose. Combining an `HTMLButtonBehavior` (activation) with an `HTMLImageBehavior` (embedded content) produces a submit button that renders as an image, mirroring native [`<input type=image>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/image):
+
+```javascript
+class ImageButton extends HTMLElement {
+  static formAssociated = true;
+  static observedAttributes = ['src', 'alt'];
+  static behaviors = [HTMLButtonBehavior, HTMLImageBehavior];
+
+  #internals;
+  #image;
+
+  constructor() {
+    super();
+    this.#internals = this.attachInternals();
+    // HTMLButtonBehavior defaults to type 'submit', so it needs no configuration.
+    this.#image = this.#internals.behaviors.get(HTMLImageBehavior);
+  }
+
+  attributeChangedCallback(name, _old, value) {
+    if (name === 'src') {
+      this.#image.src = value;
+    }
+    if (name === 'alt') {
+      this.#image.alt = value;
+    }
+  }
+}
+customElements.define('image-button', ImageButton);
+```
+
+```html
+<form action="/search">
+  <image-button src="/go.png" alt="Search"></image-button>
+</form>
+```
+
+On activation the host submits the form (from `HTMLButtonBehavior`) while rendering the image and exposing its `alt` text as the accessible name (from `HTMLImageBehavior`). The two behaviors are in different categories, so the platform allows the combination.
+
+Because the category is the base class, the platform decides compatibility with a membership test (`behavior instanceof ActivationBehavior`). Adding a new behavior only requires placing it under the right base; it does not require enumerating its compatibility with every existing behavior.
+
+#### `ActivationBehavior`
+
+`ActivationBehavior` corresponds to the DOM standard's [activation behavior](https://dom.spec.whatwg.org/#eventtarget-activation-behavior): the algorithm an `EventTarget` runs when a `click` is dispatched to it and not canceled. The DOM standard pairs it with two optional hooks, [legacy-pre-activation behavior](https://dom.spec.whatwg.org/#eventtarget-legacy-pre-activation-behavior) and [legacy-canceled-activation behavior](https://dom.spec.whatwg.org/#eventtarget-legacy-canceled-activation-behavior), which native checkbox and radio use to set checkedness before listeners run and restore it if the event is canceled.
+
+`ActivationBehavior` provides:
+
+- Participation in the activation dispatch path (click, keyboard activation, and `element.click()`), honoring `preventDefault()` and `stopPropagation()`.
+- The `legacy-pre-activation behavior` and `legacy-canceled-activation behavior` hooks (no-ops unless overridden).
+
+Each concrete behavior supplies the rest:
+
+- What the element does when activated (the activation algorithm).
+- Its implicit ARIA role default.
+- Its focusability default.
+- Its keyboard-activation specifics.
+- Its own state and the protocol surface it exposes.
+
+Mapping a few native patterns onto categories helps validate the model:
+
+| Native pattern | Behavior | Category | Notes |
+|---|---|---|---|
+| `<button>` | `HTMLButtonBehavior` | activation | submit/reset/button via `type`; invoker surface; form participation. |
+| `<a href>` | `HTMLAnchorBehavior` | activation | Role `"link"`. |
+| `<label>` | `HTMLLabelBehavior` | activation | `for`-attribute association and focus delegation, activation delegates a click to the labeled control, no implicit role, not focusable. |
+| `<input type=radio>` | `HTMLRadioGroupBehavior` | activation | `name`-based mutual exclusion; needs the `legacy-pre-activation`/`legacy-canceled-activation` hooks for the check-then-restore dance. This is the concrete reason the base exposes those hooks. |
+| `<input type=checkbox>` | `HTMLCheckboxBehavior` | activation | An independent on/off toggle, using the same `legacy-pre-activation`/`legacy-canceled-activation` hooks. |
+| `<img>` | `HTMLImageBehavior` | embedded content | Shown to demonstrate composition (a clickable image). |
+
+Of these, `HTMLButtonBehavior`, `HTMLLabelBehavior`, and `HTMLRadioGroupBehavior` are the near-term candidates for Future work. `HTMLAnchorBehavior` and `HTMLImageBehavior` appear only to illustrate the category rules (a same-category conflict and a cross-category composition); they are not behaviors this proposal defines. The embedded-content category corresponds to the HTML spec's [embedded content](https://html.spec.whatwg.org/multipage/dom.html#embedded-content-2).
+
+### Duplicate behaviors
+
+Listing the same behavior class twice in `static behaviors` (for example, `static behaviors = [HTMLButtonBehavior, HTMLButtonBehavior]`) throws a `TypeError` at `customElements.define()` time. A host has at most one instance of each behavior class. This is the degenerate case of the [category rule](#behavior-categories-and-composition): two behaviors of the same category are mutually exclusive, and two instances of the same class are trivially the same category.
+
 ### HTMLButtonBehavior
 
-This alternative introduces `HTMLButtonBehavior`, which mirrors native `<button>`.
+This alternative introduces `HTMLButtonBehavior`, a concrete behavior in the activation category (`class HTMLButtonBehavior extends ActivationBehavior`) that mirrors native `<button>`.
 
 - The behavior has a `type` property (`'submit'` (default), `'reset'`, or `'button'`) that selects the active button mode. The `type` is mutable for the life of the behavior.
 - User activation (click, Enter, Space, implicit submission) reaches the behavior through the same DOM event-dispatch path as native elements.
@@ -90,7 +206,7 @@ Each behavior exposes properties and methods from its corresponding native eleme
 
 **Properties:**
 - `type` - selects the active button mode (`'submit'`, `'reset'`, or `'button'`); defaults to `'submit'`.
-- `disabled` - The element is effectively disabled if either `behavior.disabled` is `true` or the element is disabled via attribute or is a descendant of `<fieldset disabled>` ([spec](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled)).
+- `disabled` - read-only. Reflects whether the element is disabled, which is determined by the standard form-control mechanism (the `disabled` attribute or a `<fieldset disabled>` ancestor, [spec](https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled)) and surfaced through `ElementInternals`. This proposal does not add a separately settable per-behavior disabled state.
 - `form` - read-only, delegates to `ElementInternals.form`. Form ownership only affects activation when `type` is `'submit'` or `'reset'`.
 - `name`, `value` - submitter name and value. Read on submission (`type === 'submit'`).
 - `formAction`, `formEnctype`, `formMethod`, `formNoValidate`, `formTarget` - submission overrides. Read on submission (`type === 'submit'`).
@@ -133,10 +249,6 @@ Setting `behavior.type` to an unknown string does not throw. The behavior coerce
 
 Changing `behavior.type` between events of a single interaction (for example, between `mousedown` and `mouseup`, or between `keydown` and `keyup` on a key activation) queues the change. The change applies at end-of-interaction, between event tasks. This mirrors how the platform already handles `type` mutations on a native `<button>` during click dispatch.
 
-### Duplicate behaviors
-
-Listing the same behavior class twice in `static behaviors` (for example, `static behaviors = [HTMLButtonBehavior, HTMLButtonBehavior]`) throws a `TypeError` at `customElements.define()` time. A host has at most one instance of each behavior class.
-
 ### API design
 
 Authors declare behaviors via a static class property; the platform creates and exposes the instances: