Make attachShadow() use the global registry by default

[annevk]

This significantly improves the reuse of custom elements in different registries.

Tests: web-platform-tests/wpt#55248.

Fixes #1407.

• At least two implementers are interested (and none opposed):
  • WebKit
  • Gecko
• Tests are written and can be reviewed and commented upon at:
  • See above.
• Implementation bugs are filed:
  • Chromium: See SCER bug.
  • Gecko: See SCER bug.
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=300027
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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[keithamus]

Yes +1; consider Mozilla interested.

[mfreed7]

This significantly improves the reuse of custom elements in different registries.

Tests: web-platform-tests/wpt#55248.

Fixes #1407.

Sounds reasonable to me. But I'd love if @dandclark could also take a look.

• MDN issue is filed: N/A

Quick thing: is there already an MDN issue filed for SCER overall? The existing page for attachShadow() doesn't include ShadowRootInit/customElementRegistry at all.

[annevk]

The original DOM PR (and HTML PR) has all the relevant pointers: #1341.