[css-ui] Restrict appearance: button more

[zcorpan]

Per web compat analysis [1], the button value can be restricted to

• a
• button
• input type=button/reset/submit
• drop-down box select

Part of #3024

Tests: TODO

[1] #3024 (comment)

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cc @tkent-google @foolip @MatsPalmgren @emilio @jwatt

[emilio]

How does this restriction work, is it at used value time? (looks like it from the spec text)

[zcorpan]

Yes. Keywords don't resolve/compute to other keywords, just "act as if".

[frivoal]

Looks good to me, but for one thing: I am a little surprised there's not web-compat need for button to have an effect on arbitrary divs. Did the compat study look include plain (non replaced, non-control) elements in "others" that and conclude it was fine, or was this left out?

[zcorpan]

I think "others" included any element.

@tkent-google which elements does Chromium currently allow to have button appearance?

[tkent-google]

I think "others" included any element.

@tkent-google which elements does Chromium currently allow to have button appearance?

Chrome 79 stable: button works with arbitrary elements except for <select> and <a>. We haven't received significant site-compatibility issues, and I think we can continue to exclude <select> and <a>.

Chrome 80 beta: button will work only with <button> and <input type=button/color/reset/submit>. It's still in Beta. We don't know whether wen can keep this behavior or not yet.

[frivoal]

The discussion has moved on since this PR, and #5174 has the latest on this topic.