Reducing ISTF spec to a CSS subset for consistency

CSS has lots of inconsistencies, driven by DX they thought they are optimizing. For e.g. url can be defined as a CSS string, as a url token and as a url function.

In ISTF we
1. don't need many ways to describe the same
2. want to reduce complexity for transformation and strinfigification steps

Lets discuss here all possible things we would want to describe in one consistent way.

Start by reviewing the ISTF specification and the CSS representations mentioned in the issue, including strings, URL tokens, and URL functions. Catalog other cases where CSS offers multiple ways to describe the same value, then use the issue discussion to establish a consistent subset. Done means the proposed reductions are agreed and documented in the specification.

Written by the indexing model from the issue text.