Make a '::distributed' pseudo element the first-ever client who can accept a relative selector.
The spec of relative selectors is:
'::distributed' can be categorized as the following use case mentioned in the spec:
> if the reference element set consists of only a virtual scoping root:
This patch hasn't implemented the following case, which is now treated as a parsing error. I'll address that in a follow-up patch.
> 2. Otherwise, if the selector starts with any combinator other than the descendant combinator, change the selector to ‘:not(*)’.
I also have a plan to support relative selectors in other use cases such as Element.find(relative_selectors). But today is not the day.