CSSMin: Compact CSS style definitions

This class can be used to parse and compact CSS style definitions.

It can take a string with CSS style definitions and removes all comments, empty blocks, empty declarations and whitespace that is not necessary.

It can also rewrite declaration values to a shorter notation when possible.

It also supports rewriting CSS Level 3 properties to their browser specific counterparts, as well simulate CSS variables.

Optionally it can preserve URLs in the style definitions.