Support ElementInternals via IDL extended attribute reflection

[lukewarlow]

What is the issue with the HTML Standard?

The recently introduced [Reflect] etc covers a large portion of the uage of content attribute reflection, with the exception of two main things. Enumerated attributes that are limited to only known values (see #11455 for a solution), the other thing is reflection supporting ElementInternals (in practice I think this is only used on https://w3c.github.io/aria/#dom-ariamixin).

We should solve the element internals use case so that the new IDL reflection can be used on the AriaMixin interface too.

[ReflectSupportElementInternals] could work as a new primary reflection extended attribute but it's rather long winded?

It's not immediately clear to me why this special flag is even necessary? I would have assumed it could be inferred by whether the attribute being reflected is on the ElementInternals interface (which these are via ElementInternals includes AriaMixing)

[annevk]

Yeah, we should make [Reflect] et al. branch on where they are defined. This should also allow us to remove "support ElementInternals".

cc @rahimabdi

[lukewarlow]

Am I right in thinking that because the mixin is in both places we can just delete the support ElementInternals flag entirely?

The reflected target will be ElementInternals automatically and it will all just work?

I don't know that [Reflect] et al even need any updates?

From my reading the support ElementInternals flag seems to be for if you want to do the below and have it automagically apply to ElementInternals?

interface Element {
    [Reflect] attribute DOMString fooBar;
}

[annevk]

Yeah upon re-reading I think that is correct. It's there so you don't have to say "reflect" twice in prose, but that prose is gone now.

So once we've migrated away from using "reflect" in prose for all ElementInternals IDL attributes, we can remove that paragraph.

[lukewarlow]

Okay sounds good. I'll make a PR to update AriaMixin to use the new syntax and then one to HTML to remove that.