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.

ISSUES: Record pseudoclass progress.

Correct legacy :before/:after syntax.

Support lowering single-arg psuedoclasses.

Implement psuedoclass rewrite rules.

ISSUES: Plan psuedoclass lowering.

ISSUES: Text properties *mostly* implemented, upstreamed from Rhapsode core.

Substitute in counters for any CSS textual property, not just `content`.

Will be required by Rhapsode for navigational markers.

Implement style tree flattening utility.

This will be useful in Rhapsode for making sure voice synthesizers continue to
have full voice features information as the user jumps around the document.
And it will be useful for visual browsers in transitioning from layout to render.

Implement whitespace collapse, togglable via white-space: pre/normal;

Abstract Tree traversal (commited out-of-order).

Parse & prepare to handle white-space: pre;

Implement CSS3 counters.

Parse CSS counters.

Fix compilation of Assets StyleSheets.

Optimize: use HashMap.Lazy for cascade.

Provide utilities for rewriting CSS-referenced URLs.