Allow any element to be the child of a custom element (parser weirdness permitting)

[w3cbot]

This is a tracker issue. Only discuss things here if they are a11y group internal meta-discussions about the issue. Contribute to the actual discussion at the following link:

§ https://github.com/whatwg/html/issues/10722

[stevefaulkner]

comment added https://github.com/whatwg/html/issues/10722#issuecomment-2447933745