Support for relational pseudo-class :has()

[redapple]

CSS Selectors Level 4 (still in draft) introduce the :has() pseudo-class:

The relational pseudo-class, :has(), is a functional pseudo-class taking a relative selector list as an argument. It represents an element if any of the relative selectors, when absolutized and evaluated with the element as the :scope elements, would match at least one element.

For example, the following selector matches only <a> elements that contain an <img> child:
a:has(> img)
The following selector matches a <dt> element immediately followed by another <dt> element:
dt:has(+ dt)

Although no browser seems to be supporting this yet, it looks here to stay (I may be wrong).

It would be interesting to support this to get a bit more flexibility on predicates (e.g. testing children elements).