Add utils for implementing :target-within, ensure unlayered styles take precedance.

Parse namespace selectors.

Compute correct specificity for :is() & :not() pseudoclasses.

Improve documentation, release version 1.2

Rework psuedoelement infrastructure so they can be their own boxes.

Add support for psuedoelements/classes.

Code cleanliness fixes.

Switch to new namespace for consistency with other modules like css-syntax.

Draft alternate API.