Use "self" in select position-try-fallbacks

[josepharhar]

This was decided by the CSSWG here:
w3c/csswg-drafts#13738

Corresponding CSS spec change:
w3c/csswg-drafts#13778

• At least two implementers are interested (and none opposed):
  • Chromium
  • WebKit
• Tests are written and can be reviewed and commented upon at:
  • Prepend "self" to customizable select position-try-fallbacks web-platform-tests/wpt#59080
• Implementation bugs are filed:
  • Chromium: Already implemented
  • Gecko: [css-forms-1] does select use correct writing-mode for positioning? mozilla/wg-decisions#2420
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=313157
• Corresponding HTML AAM & ARIA in HTML issues & PRs: not needed
• MDN issue is filed: not needed
• 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 )

[annevk]

I'll create a WebKit bug.