Modified superscript and subscript text styles to be mutually exclusive.

[martnpaneq]

🚀 Summary

The superscript and subscript text styles are now mutually exclusive: applying one to text that already has the other replaces it, matching the behavior of common word processors. The previous behavior, where both attributes could coexist on the same text, can be restored by setting config.basicStyles.superscript.allowNesting or config.basicStyles.subscript.allowNesting to true, which is useful for content such as isotope notation or tensor indices.

---

📌 Related issues

• Closes Superscript and subscript can be active at the same time (formatting conflict) #20000

---

💡 Additional information

Screen.Recording.2026-04-28.at.12.31.35.mov

---

🧾 Checklists

Use the following checklists to ensure important areas were not overlooked.
This does not apply to feature-branch merges.
If an item is not relevant to this type of change, simply leave it unchecked.

Author checklist

• Is the changelog entry intentionally omitted? (Changelog entry included.)
• Is the change backward-compatible? (No, but it is treated as a fix. It changes the behaviour of superscript and subscript to be aligned with other solutions on the market.)
• Have you considered the impact on different editor setups and core interactions? (e.g., classic/inline/multi-root/many editors, typing, selection, paste, tables, lists, images, collaboration, pagination) (Should not impact other editor setups in unpredicted way.)
• Has the change been manually verified in the relevant setups? (Verified in a manual test.)
• Does this change affect any of the above? (Not relevant.)
• Is performance impacted? (Performance not impacted.)
• Is accessibility affected? (Accessibility not affected.)
• Have tests been added that fail without this change (against regression)? (Appropriate tests added.)
• Have the API documentation, guides, feature digest, and related feature sections been updated where needed? (Documentation updated.)
• Have metadata files (ckeditor5-metadata.json) been updated if needed? (Not relevant.)
• Are there any changes the team should be informed about (e.g. architectural, difficult to revert in future versions or having impact on other features)? (This change does not have a big impact.)
• Were these changes documented (in Logbook)?

Reviewer checklist

• PR description explains the changes and the chosen approach (especially when performance, API, or UX is affected).
• The changelog entry is clear, user‑ or integrator-facing, and it describes any breaking changes.
• All new external dependencies have been approved and mentioned in LICENSE.md (if any).
• All human-readable, translateable strings in this PR been introduced using t() (if any).
• I manually verified the change (e.g., in manual tests or documentation).
• The target branch is correct.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 44ef9f6. Configure here.}

[jacek-cke]

LGTM

[pomek]

Edge case, but what would happen when both the subscript and superscript attributes are defined in the data on the editor load?

<p>
  Test: <sup><sub>2</sub></sup>.
</p>

I am wondering about the alternative approach:

Use a schema to control both attributes. This way, we do not have to provide additional configuration options. Instead, our docs can share a code snippet that would turn off this check.

By adding a postfixer, this approach resolves the potential issue with invalid HTML when creating the editor.

One caveat: we need to decide which attribute is more important. For the following conflict: <sup><sub>2</sub></sup>, do we expect that editor keeps

• <sup>2</sup>, or
• <sub>2</sub>

Maybe we can solve it at the upcast level, but I am not sure.