[forms] Remove anonymous button content-box, and use CSS rather than magic for input / button block axis centering.

[emilio]

This defines the (previously undefined, afaict) unsafe center alignment of text inputs and textarea, and makes the changes discussed by the CSS Working Group in w3c/csswg-drafts#14190.

• At least two implementers are interested (and none opposed):
  • Gecko
  • Blink
  • WebKit was supportive during the CSSWG meeting, see minutes.
• Tests are written and can be reviewed and commented upon at:
  • https://phabricator.services.mozilla.com/D312564
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/545527313
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2055599
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=321599
• 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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/rendering.html ( diff )

[emilio]

@zcorpan / @annevk the thing I'm not sure about how to best define is the select:not(:listbox) thing. Any pointers?