Add UA stylesheet for <select> base appearance

[josepharhar]

This PR adds a stylesheet for customizable select which applies when the select element has base appearance.

Discussion here: w3c/csswg-drafts#10857

Customizable select proposal: #9799

• At least two implementers are interested (and none opposed):
  • Chrome
  • Mozilla: Customizable select element mozilla/standards-positions#1060
  • WebKit: Customizable select element WebKit/standards-positions#386
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select/
• Implementation bugs are filed:
  • Chromium: Already implemented
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1944403
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=286642
• MDN issue is filed: Customizable select element mdn/content#37857

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

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/infrastructure.html ( diff )
/rendering.html ( diff )

[annevk]

Where do we define what base computes to when it's a list box?

[josepharhar]

In this iteration I'm not proposing supporting base appearance on list box rendering. In chromium, I am making it do the same thing as appearance:auto, and our plan to do forward compat is to add a CSS @supports rule when appearance:base is ready for list box rendering.

[annevk]

@nt1m have you had a chance to review this?

[josepharhar]

I just updated this PR to match the latest styles from the CSS issue.

I'm not sure what we should say about properties that are supposed to be inherited, like color and font. Should we explicitly say color:inherit? Should we have some text which says that all properties not listed here should be whatever their unset value is? Should we just say that other UA styles should not apply?

There is a bunch of UA styles in chromium I have to undo by adding extra CSS, and while I don't want to list all those properties specifically in this spec I also think it would be a good idea to mention that when implementing this feature, you have to undo those.

[domenic]

I think we need to list all those properties explicitly in the spec! Why would we not?

[annevk]

I think @josepharhar is describing Chromium's implementation of "base appearance", as in principle "base appearance" would start from an unstyled element so you would not have to undo styling, unless it was some global style.

[domenic]

I see. That's probably covered by the CSS spec for appearance: base then?

[josepharhar]

Agreed! I'll omit the properties which have their default.