Specify [LegacyWindowAlias=HTMLDocument] on Document

[TimothyGu]

In https://html.spec.whatwg.org/multipage/window-object.html#the-window-object there is:

For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named HTMLDocument whose value is the Document interface object.

I'd like to get rid of that using the still relatively new [LegacyWindowAlias] extended attribute. However, Web IDL forbids that extended attribute from being used on partial interface definitions, and thus I had to open the PR here rather than as HTML. As much as I hate putting more HTML-specific things in DOM, I think this is minor enough to be okay.

As soon as this gets merged I'll open a PR to HTML removing that paragraph.

/cc @tobie

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[tobie]

LGTM

[annevk]

So #221 and #278 are not resolved yet. It's unclear if any engine actually implements this, so I'm not sure it's a good idea to make this change as it might lead folks to think we made some kind of decision here.

[domenic]

I think it'd still be a good idea to add this here as it's a non-normative change that makes things easier to understand while reading the spec. (And also easier for any implementations like Servo or jsdom that implement it exactly.)

We could separately add a warning about #221 and #278, but I'd like to see this merged if possible.

[annevk]

When I talked it through with @foolip it sounds like the solution will be that we have HTMLDocument as a distinct empty interface (similar to XMLDocument) and HTML will need to carefully define when it gets allocated. Document will still define all the joint members. And I guess we get to keep SVGDocument too?

[foolip]

Only Safari has SVGDocument (as an alias of XMLDocument) so we don't need that.

[ExE-Boss]

Unfortunately, it looks like whatwg/html#4792 might throw a spanner into this.

[annevk]

Closing as it seems even more unlikely we'll end up using this at this point.