Add hook for callers to run their own tests against attributes.

Expand :is/:where lowering.

Codestyle: remove unnecessary parens.

Move :root handling into interpretor.

Start testing counters implementations, fix syntactic shorthand lowering for counter[s]() functions.

Evaluate attribute namespace selectors.

Parse namespace selectors.

Rewrite trivial cases of :where & :is.

This is mostly for the psuedoselector rewrites performed in Data.CSS.Preprocessor.PsuedoClasses

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

Implement :nth-child() & :nth-of-type().

I don't think I'll implement :nth-last-child() & :nth-last-of-type() until I'm shown their value,
Haskell Stylist doesn't yet have access to the data to support them.

Implement :is, :where, & :not functional pseudoclasses.

This is needed by some of the lowerings for other pseudoclasses.
TODO correctly compute specificity.

Prepare to interpret functional pseudoclasses.

Optimize: use HashMap.Lazy for cascade.

Improve documentation, release version 1.2

Write reference documentation.

Test rules capturing of ConditionalStyleSheet.

Code cleanliness: drop unneeded ' from Data.CSS.Style.Cascade.dispatch'

Test CSS variables.

Implement var() via a PropertyParser decorator.

Expose hooks for implementing var().

The tricky part here is around CSS inheritance, it works naturally in the existing cascade implementation.