Render an option's label attribute when not using base appearance

[annevk]

Render an option's label attribute when not using base appearance

Defining customizable <select> in 172cccf replaced "displaying the
element's label" with "displaying the result of collect option text
given the option and true", which never consults the label content
attribute and strips and collapses whitespace. As written,
<option label="foo">bar</option> displays "bar", and an option whose
label attribute is a single space displays nothing.

Blink, Gecko, and WebKit instead display the label attribute's value
as-is when it is not the empty string, and the option's text otherwise.
That is what option-label-whitespace.html and
option-label-whitespace-2.html in
html/rendering/replaced-elements/the-option-element/ test.

The rendering section could not simply reference the option's label
because it needs img alt text included. So the label is now an algorithm
that takes the same includeAltText boolean as collect option text,
mirroring "get an optgroup element's label": the label IDL attribute
getter passes false and rendering passes true.

Base appearance is unaffected; the option contents slot and option label
element already work this way.

While there, add a missing "the" to the sentences introducing the label
and value attributes.

Fixes #10955.

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The prose this restores was lost in #10548 and stayed lost through #12201; base appearance, defined by those same PRs, already gets it right.

• At least two implementers are interested (and none opposed) — all three already ship this behavior, so this PR is catching the standard up:
  • Chromium: https://chromium-review.googlesource.com/c/chromium/src/+/6215230 and https://chromium-review.googlesource.com/c/chromium/src/+/6354924
  • Gecko: has always behaved this way (it was the engine matching the pre-Define customizable <select> #10548 text)
  • WebKit: https://commits.webkit.org/311421@main
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/html/rendering/replaced-elements/the-option-element/option-label-whitespace.html
  • https://github.com/web-platform-tests/wpt/blob/master/html/rendering/replaced-elements/the-option-element/option-label-whitespace-2.html
• Implementation bugs are filed: not needed, Chromium, Gecko, and WebKit all pass the tests above.
• Corresponding HTML AAM & ARIA in HTML issues & PRs: not needed, this is rendering only.
• MDN issue is filed: not needed, no observable change from what browsers already do.
• 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 )

[keithamus]

LGTM.