Check contrast of CSS ::highlight pseudo classes

It's possible to use `::highlight()` in CSS to change the foreground and background colors of text. Axe-core should look at this when testing, since it can result both in false positives and false negatives.

I've never seen anyone use this, so it's mostly a theoretical problem. Here's a codepen with how that might work:
https://codepen.io/wilcofiers/pen/xxorwOe