Clarify behavior of calling attachShadow on a declarative shadowRoot

[sorvell]

What is the issue with the HTML Standard?

Calling attachShadow on a declaratively created shadowRoot does not set the values supplied in the ShadowRootInit. It appears to be expected based on the spec that you should be able to configure shadowRoot properties other than mode. All browsers currently behave consistently so this likely needs clarification in the spec.

See this reproduction.

[annevk]

This is already tested and clear. See step 4 of https://dom.spec.whatwg.org/#concept-attach-a-shadow-root

[sorvell]

Got it, thanks for clarifying. I missed the return at step 4.3.3.

Based on this behavior, I think it definitely makes sense to expose as much configuration as possible on the declarative shadow template, see #12267.