diff --git a/packages/blocks/README.md b/packages/blocks/README.md
index ee80df05735c50..29ca97aa183c06 100644
--- a/packages/blocks/README.md
+++ b/packages/blocks/README.md
@@ -97,6 +97,7 @@ _Parameters_
 -   _blockTypeOrName_ `string | BlockType`: Block type or name.
 -   _innerHTML_ `string | Node`: Raw block content.
 -   _attributes_ `Record< string, unknown >`: Known block attributes (from delimiters).
+-   _parsedBody_ `Node | null`: Optional pre-parsed DOM node for innerHTML. When provided, the internal HTML parse is skipped. Useful for sharing a single parse across multiple callers operating on the same innerHTML string (e.g. block validation, deprecation iteration). Note: hpq uses a single shared document body, so a parsed node held across other parses will be detached. Detached nodes still respond correctly to attribute and class reads.
 
 _Returns_
 
diff --git a/packages/blocks/src/api/parser/apply-block-deprecated-versions.ts b/packages/blocks/src/api/parser/apply-block-deprecated-versions.ts
index e4f5ce434f6adc..01957e8eadee31 100644
--- a/packages/blocks/src/api/parser/apply-block-deprecated-versions.ts
+++ b/packages/blocks/src/api/parser/apply-block-deprecated-versions.ts
@@ -22,18 +22,24 @@ function stubFalse(): boolean {
  * deprecated migrations applied, or the original block if it was both valid
  * and no eligible migrations exist.
  *
- * @param block     Parsed and invalid block object.
- * @param rawBlock  Raw block object.
- * @param blockType Block type. This is normalize not necessary and
- *                  can be inferred from the block name,
- *                  but it's here for performance reasons.
+ * @param block      Parsed and invalid block object.
+ * @param rawBlock   Raw block object.
+ * @param blockType  Block type. This is normalize not necessary and
+ *                   can be inferred from the block name,
+ *                   but it's here for performance reasons.
+ * @param parsedBody Pre-parsed DOM body for `block.originalContent`, if
+ *                   available. Shared across loop iterations (passed to
+ *                   `getBlockAttributes` to skip re-parsing, and to
+ *                   `applyBuiltInValidationFixes` which extracts the root
+ *                   element from it for the per-fix attribute reads).
  *
  * @return Migrated block object.
  */
 export function applyBlockDeprecatedVersions(
 	block: Block,
 	rawBlock: RawBlock,
-	blockType: BlockType
+	blockType: BlockType,
+	parsedBody?: Element | null
 ): Block {
 	const parsedAttributes = rawBlock.attrs ?? {};
 	const { deprecated: deprecatedDefinitions } = blockType;
@@ -79,7 +85,8 @@ export function applyBlockDeprecatedVersions(
 			attributes: getBlockAttributes(
 				deprecatedBlockType,
 				block.originalContent ?? '',
-				parsedAttributes
+				parsedAttributes,
+				parsedBody
 			),
 		};
 
@@ -90,7 +97,8 @@ export function applyBlockDeprecatedVersions(
 		if ( ! isValid ) {
 			migratedBlock = applyBuiltInValidationFixes(
 				migratedBlock,
-				deprecatedBlockType
+				deprecatedBlockType,
+				parsedBody
 			);
 			[ isValid ] = validateBlock( migratedBlock, deprecatedBlockType );
 		}
diff --git a/packages/blocks/src/api/parser/apply-built-in-validation-fixes.ts b/packages/blocks/src/api/parser/apply-built-in-validation-fixes.ts
index dd1b4feca76042..c3b841edbeda91 100644
--- a/packages/blocks/src/api/parser/apply-built-in-validation-fixes.ts
+++ b/packages/blocks/src/api/parser/apply-built-in-validation-fixes.ts
@@ -23,25 +23,41 @@ const ANCHOR_ATTR_SCHEMA: BlockAttribute = {
  * Attempts to fix block invalidation by applying build-in validation fixes
  * like moving all extra classNames to the className attribute.
  *
- * @param block     block object.
- * @param blockType Block type. This is normalize not necessary and
- *                  can be inferred from the block name,
- *                  but it's here for performance reasons.
+ * @param block      block object.
+ * @param blockType  Block type. This is normalize not necessary and
+ *                   can be inferred from the block name,
+ *                   but it's here for performance reasons.
+ * @param parsedBody Pre-parsed body element of `block.originalContent`, if
+ *                   available. When provided, the fixes read attributes
+ *                   directly off the body's first element child instead of
+ *                   re-parsing originalContent for each fix.
  *
  * @return Fixed block object
  */
 export function applyBuiltInValidationFixes(
 	block: Block,
-	blockType: BlockType
+	blockType: BlockType,
+	parsedBody?: Element | null
 ): Block {
 	const { attributes, originalContent } = block;
 	let updatedBlockAttributes = attributes;
 
+	// Extract the root element once: every fix below reads attributes off the
+	// block's outermost element. `undefined` here means "no pre-parsed body
+	// supplied" — the fixes will fall back to parsing originalContent
+	// themselves. `null` means "we have a parsed body but it has no element
+	// child" (e.g. text-only innerHTML).
+	const rootElement =
+		parsedBody !== undefined
+			? parsedBody?.firstElementChild ?? null
+			: undefined;
+
 	// Fix block invalidation for className attribute.
 	updatedBlockAttributes = fixCustomClassname(
 		attributes,
 		blockType,
-		originalContent ?? ''
+		originalContent ?? '',
+		rootElement
 	);
 	// Fix block invalidation for ariaLabel attribute.
 	updatedBlockAttributes = fixGlobalAttribute(
@@ -50,7 +66,8 @@ export function applyBuiltInValidationFixes(
 		originalContent ?? '',
 		'ariaLabel',
 		'data-aria-label',
-		ARIA_LABEL_ATTR_SCHEMA
+		ARIA_LABEL_ATTR_SCHEMA,
+		rootElement
 	);
 	// Fix block invalidation for anchor attribute.
 	updatedBlockAttributes = fixGlobalAttribute(
@@ -59,7 +76,8 @@ export function applyBuiltInValidationFixes(
 		originalContent ?? '',
 		'anchor',
 		'data-anchor',
-		ANCHOR_ATTR_SCHEMA
+		ANCHOR_ATTR_SCHEMA,
+		rootElement
 	);
 
 	return {
diff --git a/packages/blocks/src/api/parser/fix-custom-classname.ts b/packages/blocks/src/api/parser/fix-custom-classname.ts
index 795490ec34e04e..bed833fb4bc18d 100644
--- a/packages/blocks/src/api/parser/fix-custom-classname.ts
+++ b/packages/blocks/src/api/parser/fix-custom-classname.ts
@@ -3,15 +3,18 @@
  */
 import { hasBlockSupport } from '../registration';
 import { getSaveContent } from '../serializer';
-import { parseWithAttributeSchema } from './get-block-attributes';
-import type { BlockAttribute, BlockType } from '../../types';
+import { parseHtml } from './get-block-attributes';
+import type { BlockType } from '../../types';
 
-const CLASS_ATTR_SCHEMA: BlockAttribute = {
-	type: 'string',
-	source: 'attribute',
-	selector: '[data-custom-class-name] > *',
-	attribute: 'class',
-};
+function splitClassName( className: unknown ): string[] {
+	return typeof className === 'string' && className
+		? className.trim().split( /\s+/ )
+		: [];
+}
+
+function getElementClasses( element: Element | null ): string[] {
+	return splitClassName( element?.getAttribute( 'class' ) );
+}
 
 /**
  * Given an HTML string, returns an array of class names assigned to the root
@@ -22,12 +25,8 @@ const CLASS_ATTR_SCHEMA: BlockAttribute = {
  * @return Array of class names assigned to the root element.
  */
 export function getHTMLRootElementClasses( innerHTML: string ): string[] {
-	const parsed = parseWithAttributeSchema(
-		`<div data-custom-class-name>${ innerHTML }</div>`,
-		CLASS_ATTR_SCHEMA
-	) as string | undefined;
-
-	return parsed ? parsed.trim().split( /\s+/ ) : [];
+	const root = ( parseHtml( innerHTML ) as Element )?.firstElementChild;
+	return getElementClasses( root );
 }
 
 /**
@@ -39,13 +38,17 @@ export function getHTMLRootElementClasses( innerHTML: string ): string[] {
  * @param blockAttributes Original block attributes.
  * @param blockType       Block type settings.
  * @param innerHTML       Original block markup.
+ * @param rootElement     Pre-parsed root element of innerHTML, if available.
+ *                        When provided, avoids re-parsing innerHTML to read
+ *                        the actual classes.
  *
  * @return Filtered block attributes.
  */
 export function fixCustomClassname(
 	blockAttributes: Record< string, unknown >,
 	blockType: BlockType,
-	innerHTML: string
+	innerHTML: string,
+	rootElement?: Element | null
 ): Record< string, unknown > {
 	if ( ! hasBlockSupport( blockType, 'customClassName', true ) ) {
 		return blockAttributes;
@@ -59,8 +62,15 @@ export function fixCustomClassname(
 	const { className: omittedClassName, ...attributesSansClassName } =
 		modifiedBlockAttributes;
 	const serialized = getSaveContent( blockType, attributesSansClassName );
+	// `getHTMLRootElementClasses` writes the rendered output into hpq's
+	// shared document body. Callers that pass `rootElement` are insulated by
+	// the deep clone of `parsedBody` in `parseRawBlock`; without that clone,
+	// this parse would mutate the shared body under our feet.
 	const defaultClasses = getHTMLRootElementClasses( serialized );
-	const actualClasses = getHTMLRootElementClasses( innerHTML );
+	const actualClasses =
+		rootElement !== undefined
+			? getElementClasses( rootElement )
+			: getHTMLRootElementClasses( innerHTML );
 
 	const customClasses = actualClasses.filter(
 		( className ) => ! defaultClasses.includes( className )
diff --git a/packages/blocks/src/api/parser/fix-global-attribute.ts b/packages/blocks/src/api/parser/fix-global-attribute.ts
index e8d555aaa9ec74..790833c5619431 100644
--- a/packages/blocks/src/api/parser/fix-global-attribute.ts
+++ b/packages/blocks/src/api/parser/fix-global-attribute.ts
@@ -37,6 +37,9 @@ export function getHTMLRootElement(
  * @param supportKey      The block support key to check and attribute key to set.
  * @param dataAttribute   The data attribute name to use as wrapper.
  * @param attributeSchema The attribute schema configuration.
+ * @param rootElement     Pre-parsed root element of innerHTML, if available.
+ *                        When provided, the attribute is read directly off the
+ *                        root element, avoiding a wrap-and-reparse.
  *
  * @return Filtered block attributes.
  */
@@ -46,17 +49,26 @@ export function fixGlobalAttribute(
 	innerHTML: string,
 	supportKey: string,
 	dataAttribute: string,
-	attributeSchema: BlockAttribute
+	attributeSchema: BlockAttribute,
+	rootElement?: Element | null
 ): Record< string, unknown > {
 	if ( ! hasBlockSupport( blockType, supportKey, false ) ) {
 		return blockAttributes;
 	}
 	const modifiedBlockAttributes = { ...blockAttributes };
-	const attributeValue = getHTMLRootElement(
-		innerHTML,
-		dataAttribute,
-		attributeSchema
-	);
+	let attributeValue: unknown;
+	if ( rootElement !== undefined ) {
+		const attrName = attributeSchema.attribute as string | undefined;
+		attributeValue = attrName
+			? rootElement?.getAttribute( attrName ) ?? undefined
+			: undefined;
+	} else {
+		attributeValue = getHTMLRootElement(
+			innerHTML,
+			dataAttribute,
+			attributeSchema
+		);
+	}
 	if ( attributeValue ) {
 		modifiedBlockAttributes[ supportKey ] = attributeValue;
 	}
diff --git a/packages/blocks/src/api/parser/get-block-attributes.ts b/packages/blocks/src/api/parser/get-block-attributes.ts
index 8cc4ddbebe4eb5..0bd5bb3c676e66 100644
--- a/packages/blocks/src/api/parser/get-block-attributes.ts
+++ b/packages/blocks/src/api/parser/get-block-attributes.ts
@@ -259,7 +259,7 @@ export const matcherFromSource = memoize(
  *
  * @return Parsed DOM node.
  */
-function parseHtml( innerHTML: string | Node ): Node {
+export function parseHtml( innerHTML: string | Node ): Node {
 	return hpqParse( innerHTML, ( h: Node ) => h );
 }
 
@@ -287,15 +287,24 @@ export function parseWithAttributeSchema(
  * @param blockTypeOrName Block type or name.
  * @param innerHTML       Raw block content.
  * @param attributes      Known block attributes (from delimiters).
+ * @param parsedBody      Optional pre-parsed DOM node for innerHTML. When
+ *                        provided, the internal HTML parse is skipped. Useful
+ *                        for sharing a single parse across multiple callers
+ *                        operating on the same innerHTML string (e.g. block
+ *                        validation, deprecation iteration). Note: hpq uses a
+ *                        single shared document body, so a parsed node held
+ *                        across other parses will be detached. Detached nodes
+ *                        still respond correctly to attribute and class reads.
  *
  * @return All block attributes.
  */
 export function getBlockAttributes(
 	blockTypeOrName: string | BlockType,
 	innerHTML: string | Node,
-	attributes: Record< string, unknown > = {}
+	attributes: Record< string, unknown > = {},
+	parsedBody?: Node | null
 ): Record< string, unknown > {
-	const doc = parseHtml( innerHTML );
+	const doc = parsedBody ?? parseHtml( innerHTML );
 	const blockType = normalizeBlockType( blockTypeOrName );
 
 	const blockAttributes = Object.fromEntries(
diff --git a/packages/blocks/src/api/parser/index.ts b/packages/blocks/src/api/parser/index.ts
index efcbe4bfe4d1bf..e4b954707243e3 100644
--- a/packages/blocks/src/api/parser/index.ts
+++ b/packages/blocks/src/api/parser/index.ts
@@ -17,7 +17,7 @@ import { validateBlock } from '../validation';
 import { createBlock } from '../factory';
 import { convertLegacyBlockNameAndAttributes } from './convert-legacy-block';
 import { serializeRawBlock } from './serialize-raw-block';
-import { getBlockAttributes } from './get-block-attributes';
+import { getBlockAttributes, parseHtml } from './get-block-attributes';
 import { applyBlockDeprecatedVersions } from './apply-block-deprecated-versions';
 import { applyBuiltInValidationFixes } from './apply-built-in-validation-fixes';
 import type { Block, BlockType, RawBlock, ParseOptions } from '../../types';
@@ -129,11 +129,15 @@ function createMissingBlockType( rawBlock: RawBlock ): RawBlock {
  *
  * @param unvalidatedBlock
  * @param blockType
+ * @param parsedBody       Pre-parsed body element of the block's
+ *                         originalContent, if available. Threaded down to the
+ *                         built-in fixes so they don't have to re-parse.
  * @return validated block, with auto-fixes if initially invalid
  */
 function applyBlockValidation(
 	unvalidatedBlock: Block,
-	blockType: BlockType
+	blockType: BlockType,
+	parsedBody?: Element | null
 ): Block {
 	// Attempt to validate the block.
 	const [ isValid ] = validateBlock( unvalidatedBlock, blockType );
@@ -146,7 +150,8 @@ function applyBlockValidation(
 	// like custom classNames handling.
 	const fixedBlock = applyBuiltInValidationFixes(
 		unvalidatedBlock,
-		blockType
+		blockType,
+		parsedBody
 	);
 	// Attempt to validate the block once again after the built-in fixes.
 	const [ isFixedValid, validationIssues ] = validateBlock(
@@ -197,25 +202,52 @@ export function parseRawBlock(
 		return;
 	}
 
-	// Parse inner blocks recursively.
+	// Parse inner blocks recursively. This must happen *before* parsing this
+	// block's HTML — hpq uses a single shared document body, so each parse
+	// clobbers the previous one. We need this block's parsed body to remain
+	// stable through getBlockAttributes / validation / deprecation, so the
+	// recursion (which clobbers the body for each inner block) has to run
+	// first.
 	const parsedInnerBlocks = normalizedBlock.innerBlocks
 		.map( ( innerBlock ) => parseRawBlock( innerBlock, options ) )
 		// See https://github.com/WordPress/gutenberg/pull/17164.
 		.filter( ( innerBlock ) => !! innerBlock );
 
+	// Parse this block's innerHTML once and share the result with attribute
+	// extraction, validation fixes, and deprecation handling. Capture the
+	// root element eagerly: subsequent parses (e.g. of freshly serialized
+	// content inside the validation fixes) detach this element from the
+	// shared body, but detached elements still respond correctly to
+	// attribute and class reads.
+	// Parse via hpq, then deep-clone so the captured body is independent of
+	// hpq's shared document. Subsequent parses elsewhere in the pipeline
+	// (notably `fixCustomClassname`'s fallback path, which renders and
+	// re-parses save content) reset hpq's shared body — without the clone,
+	// `parsedBody` would silently change content under our feet between
+	// deprecation iterations.
+	const innerHTML = normalizedBlock.innerHTML;
+	const sharedBody = parseHtml( innerHTML ) as Element;
+	const parsedBody =
+		( sharedBody?.cloneNode( true ) as Element | null ) ?? null;
+
 	// Get the fully parsed block.
 	const parsedBlock = createBlock(
 		normalizedBlock.blockName!,
 		getBlockAttributes(
 			blockType,
-			normalizedBlock.innerHTML,
-			normalizedBlock.attrs
+			innerHTML,
+			normalizedBlock.attrs,
+			parsedBody
 		),
 		parsedInnerBlocks
 	);
-	parsedBlock.originalContent = normalizedBlock.innerHTML;
+	parsedBlock.originalContent = innerHTML;
 
-	const validatedBlock = applyBlockValidation( parsedBlock, blockType );
+	const validatedBlock = applyBlockValidation(
+		parsedBlock,
+		blockType,
+		parsedBody
+	);
 	const { validationIssues } = validatedBlock;
 
 	// Run the block deprecation and migrations.
@@ -225,7 +257,8 @@ export function parseRawBlock(
 	const updatedBlock = applyBlockDeprecatedVersions(
 		validatedBlock,
 		normalizedBlock,
-		blockType
+		blockType,
+		parsedBody
 	);
 
 	if ( ! updatedBlock.isValid ) {