Add logic to update radio button group checkedness when an input radio becomes connected

[EdgarChen]

Closes #10897.

• At least two implementers are interested (and none opposed):
  • Chromium and WebKit already behave like this.
• Tests are written and can be reviewed and commented upon at:
  • [Gecko Bug 1938489] Don’t update other radio inputs in the same radio group when a radio input is appended to a disconnected subtree web-platform-tests/wpt#50048
• Implementation bugs are filed:
  • Chromium: None
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1938489
  • WebKit: None
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): None
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): None
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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/input.html ( diff )

[EdgarChen]

cc @domfarolino @annevk

[domfarolino]

LGTM. One question: the issue mentions:

However, all browsers also update the checkedness state of elements in the same radio button group when the element becomes connected

But you also have an implementation bug filed for Firefox. Do any implementations need to do any work for this, or is this PR just bringing the spec inline with what all browsers do?