Define base appearance for list box select elements

[josepharhar]

This PR defines base appearance for select elements which are being rendered as a list box, as opposed to the prior customizable select PR which defined base appearance for drop down select elements.

Fixes #11477

• At least two implementers are interested (and none opposed):
  • Chrome
  • …
  • Standards positions:
    • Base appearance for list box select elements WebKit/standards-positions#559
    • Base appearance for list box select elements mozilla/standards-positions#1304
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select-in-page
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/357649033
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• 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 )
/rendering.html ( diff )

[lukewarlow]

Does this make sense for listbox styles? (I think probably yes but just worth checking)

[josepharhar]

Since user-select:none currently applies to the listbox inside the picker for drop-down customizable select, I think we should have it be the same here. Context: #10876