Allow the creation of null registry element and shadow root

[ja-y-son]

As part of the effort to support null custom element registry elements without shadow DOM, we're updating the spec to allow the creation of element and shadowroot to use null as a valid argument and create null registry element and shadowroot.

Fixes #1413 partially. We need another PR to support element attribute for null custom element registry in addition to #1423.

• At least two implementers are interested (and none opposed):
  • webkit
  • chromium
• Tests are written and can be reviewed and commented upon at:
  • Add new tests for customelementregistry content attribute and specifying null to createEleent and attachShadow per web-platform-tests/wpt#56108
  • Move scoped custom element registry for createElement and attachShadow web-platform-tests/wpt#56176
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/399124619
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1874414
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=302583
• 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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Preview | Diff

[ja-y-son]

@rniwa @annevk here's another PR from what we discussed.

May be a dumb question: with this change, we probably no longer want to use null as the default vaule for customElementRegistry in ShadowRootInit. In that case are we still accounting for the use case where developer passing in a ShadowRoot instance as the argument to attachShadow?

[rniwa]

You mean someone doing: host.attachShadow(anotherShadowRoot)? Given customElementRegistry property on ShadowRoot continue to return the custom element registry of the shadow root, I don't see how that'll be broken?

[sorvell]

This matches my understanding of the first set of changes proposed in #1413 (comment) and agreed to at TPAC. Good job!

[annevk]

Looks good modulo (my) nits.

[annevk]

OP is just lacking implementation bugs at this point, unless you want to add more test coverage still?

[ja-y-son]

Added the implementation bugs in OP and the test coverage should be good for now. This PR should be ready to land :)

[annevk]

While doing a final check here I noticed that the new logic would end up throwing for the combination of "is" and "customElementRegistry" way too often. While WebKit would not object to that, I doubt that was the intent. I pushed a commit to fix that (and also fix some remaining nits).

Did you not yet implement this change perhaps or do we not have good test coverage for this?

[ja-y-son]

Thanks for catching that "is" and "customElementRegistry" combination issue and fixing the nits I missed before! We've yet to implement this change and that was indeed not intended.