Update selectedcontent in selectedness setting

[josepharhar]

This PR makes sure that the contents of the selectedcontent element stay up to date when the selected option is changed in the selectedness setting algorithm.

This issue was found here: web-platform-tests/wpt#55849 (comment)

Fixes #11883 (comment)

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select/selectedcontent-mutations.html
• Implementation bugs are filed:
  • Chromium: Already implemented
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs: Not needed
• MDN issue is filed: Not needed
• The top of this comment includes a clear commit message to use.

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

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/form-elements.html ( diff )