Administration: Remove the obsolete ::-moz-focus-inner rules

[irozum]

Summary

What: Removes the ::-moz-focus-inner rules from buttons.css and editor.css entirely.

Why:

• The rules were added to remove the extra padding Firefox used to add to buttons, and the dotted inner focus border on Windows.
• Firefox 53 dropped the extra padding, and ::-moz-focus-inner itself has since been removed from Firefox on every platform.
• MDN's browser-compat-data dropped the selector in Remove obsolete -moz-image-region and -moz-focus-inner mdn/browser-compat-data#27467 (August 2025) as "removed from all browsers for more than two years".
• The rules no longer match anything, so they're dead CSS.

Not changed:

• jquery-ui-dialog.css is third-party, left untouched per the feedback on the ticket.
• The bundled themes (Twenty Thirteen through Twenty Twenty-One) also carry ::-moz-focus-inner rules. They belong to the Bundled Themes component and are better handled in a separate ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/41580

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5.5, Claude Opus 5.5
Used for: Implementation, browser-compatibility research, and PR description. Reviewed by Igor Rozum.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props irozum, afercia.

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[github-actions]

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[afercia]

@irozum thanks for the PR.
Can you please detail any reference that states that Firefox for Windows still uses ::-moz-focus-inner?
To my understanding, it si deprecated and not used any lonver, not even on Windows:
https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::-moz-focus-inner

[irozum]

Thanks @afercia, you're right. That line came from the 2017 ticket description and is no longer accurate. MDN's browser-compat-data removed ::-moz-focus-inner in mdn/browser-compat-data#27467 (Aug 2025) as "removed from all browsers for more than two years", so it doesn't match anything in current Firefox on any platform.

I've updated the PR to remove the ::-moz-focus-inner rules from buttons.css and editor.css entirely instead of simplifying them. Third-party jquery-ui-dialog.css is left untouched, as you suggested on the ticket. The bundled themes also still have these rules, but I think they're better handled in a separate ticket for the Bundled Themes component. Happy to open one if that's useful.

[afercia]

Thank you @irozum for confirming.
I'd agree the Bundled Themes should be handled in a separate ticket, please do feel free to create one!

[afercia]

I pushed a commit to remove ::-moz-focus-inner from wp-includes/css/jquery-ui-dialog.css.

The filename may be misleading but jquery-ui-dialog.css is a custom file in Core that has been updated several times.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 64333
GitHub commit: 34ecb33

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.