Intent to Implement: Shadow DOM v1

Implement new Shadow DOM APIs based on the *slots* proposal. New APIs are (including, but not limited to):

- Element.attachShadow

- New HTML Element: HTMLSlotElement

- NonDocumentTypeChildNode.assignedSlot

Note that Blink has already shipped Shadow DOM, called *v0*. See "ongoing technical constraints" section for details.

All browser vendors finally agreed on the Shadow DOM spec, called *V1*.

Firefox: In development: https://bugzilla.mozilla.org/show_bug.cgi?id=811542

Edge: Public support: https://blogs.windows.com/msedgedev/2015/07/15/microsoft-edge-and-web-components/ 

Safari: In development: https://bugs.webkit.org/show_bug.cgi?id=148695

There are still active discussions on the spec, https://github.com/w3c/webcomponents/labels/v1, however, I think it's okay to start to implement because we agreed the essential parts of v1 spec.

There is an on-going discussion how Shadow DOM v0, which Blink has already shipped, and incoming Shadow DOM v1 should interact each other. That's an issue Blink should resolve.

See https://github.com/w3c/webcomponents/issues/130 for details.

I'd like to start to implement v1 now so that I can know the feasibility. I'll keep the issue up-to-date.

Yes

No