What makes styles 'critical'?

I'm not sure how `usus` (CSS Coverage, really) classifies styles as 'critical'.
Most other tools, I believe, look at the styles needed to render the first screen (above the fold) of content.

`usus` seems to take more styles as critical?

Start by tracing how usus, described here as CSS Coverage, classifies styles as critical and compare that behavior with above-the-fold definitions. Document which styles are included and why, or clarify the expected behavior; the issue names no files or tests to run.

Written by the indexing model from the issue text.