Support `shadowrootslotassignment=` attribute

[keithamus]

WebKittens

@annevk @rniwa

Title of the proposal

HTML shadowrootslotassignment attribute

URL to the spec

whatwg/html#12267

URL to the spec's repository

https://github.com/whatwg/html

Issue Tracker URL

https://bugs.webkit.org/show_bug.cgi?id=310090

Explainer URL

No response

TAG Design Review URL

No response

Mozilla standards-positions issue URL

No response

WebKit Bugzilla URL

No response

Radar URL

No response

Description

Add support for shadowrootslotassignment= attribute.

[rniwa]

This seems fine. I suggest we mark this as "support" one week from now.