[DRAFT] Add readonly attribute to fieldset element

[teoeter]

Add readonly attribute to fieldset element, propagating read-only state to descendant form controls that support it — mirroring how fieldset disabled works.

Closes #10293

• At least two implementers are interested (and none opposed):
  • Pending
  • Pending
• Tests are written and can be reviewed and commented upon at: [DRAFT] Add tests for fieldset readonly attribute web-platform-tests/wpt#59741
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/510815051
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2038043
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=314344
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
  • HTML AAM: Map fieldset readonly state to accessibility w3c/html-aam#606
  • HTML ARIA: Allowed/expected behavior of readonly on fieldset w3c/html-aria#588
• MDN issue is filed: Document readonly attribute on <fieldset> mdn/content#44045
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

Changeset

• Adds readonly content attribute and readonly IDL attribute to HTMLFieldSetElement
• Defines concept-fieldset-readonly and concept-fe-readonly parallel to the existing disabled concepts
• Propagates readonly to input elements (where readonly applies), textarea, and form-associated custom elements - excluding legend descendants
• Adds formReadonlyCallback to form-associated custom elements, parallel to formDisabledCallback
• Adds an entry to the attributes table
• Adds an example

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