Remove custom element state when is attribute is blocked

[evilpie]

This depends on changes in #385 that haven't happened yet.

I am not really certain if there isn't some other data structure that we need to patch for this. Presumably this would be easier to specify when we do streaming parsing.

Fixes #387.

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Preview | Diff

[otherdaniel]

Looks good editorially, but I wonder whether this would execute the custom element constructor during parsing?

As I understand this, this would create the custom element, and then later remove it again. If so, I'd expect this to execute the custom element's constructor, and thus bypass sanitization in a sense.

[evilpie]

Looks good editorially, but I wonder whether this would execute the custom element constructor during parsing?

As I understand this, this would create the custom element, and then later remove it again. If so, I'd expect this to execute the custom element's constructor, and thus bypass sanitization in a sense.

No it doesn't AFAIK. The custom constructor (if not removed) is only invoked when inserting. IIRC because the custom elements registry is null for those documents we use for parsing?

You can actually try this out with div.setHTML('<div is="custom-div"></div>') in Firefox Nightly.

[evilpie]

Rebased. Should be ready for review, but can't land before #387 of course.