Support for CSS Cascade Layer import in CssOptimizer

Do you have any hints on how to run specific Tests and update the fixture data?

Also why does the regex not allow stream wrappers? It's explicitly excluded with the negative lookahead

I noticed that the current regex excludes stream wrappers. Could you clarify the reasoning behind this exclusion? Are there specific edge cases or security concerns being considered here? It might be helpful to document these in the code comments for better clarity.

Also, regarding the test cases—are there any guidelines or examples for contributors who want to run specific tests and update the fixture data? Clear instructions could encourage more contributors to test and validate this feature.