[dev-platform] Intent to Prototype & Ship: shadowrootslotassignment attribute

[Keith Cirkel]

Summary: Support for the `shadowrootslotassignment` attribute, mapping to `slotAssignment` on `attachShadow`.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2023824

Specification: https://github.com/whatwg/html/pull/12267

Standards Body: WHATWG

Platform coverage: All

Preference: dom.shadowdom.shadowRootSlotAssignment.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Supportive.

  - WebKit: Positive (https://github.com/WebKit/standards-positions/issues/631).

web-platform-tests: WPTs in https://wpt.fyi/results/shadow-dom/declarative/declarative-shadow-dom-slot-assignment.tentative.html & https://wpt.fyi/results/shadow-dom/declarative/declarative-shadow-dom-repeats-slot-assignment.tentative.html.

Template elements have a set of `shadowroot*` attributes aligning to the options bag for `attachShadow`. One omission was `shadowrootslotassignment`. I proposed adding this (https://github.com/whatwg/html/pull/12267) and it has recieved support from other vendors.

Support for this attribute landed in Firefox Nightlies yesterday, behind a flag. Once the HTML spec PR is approved and merged, I intend to turn that flag on by default. I anticipate this will result in this feature being part of the 151 release (depending, of course, on how timely the HTML spec PR is approved and merged).

If anyone has any concerns please let me know!

Happy browsing.