Feature Request: Reduce Unused CSS in Jetpack Boost

### Impacted plugin

Boost

### What

A user requested in [this thread](https://wordpress.org/support/topic/feature-request-reduce-unused-css/) to reduce unused CSS in the Boost plugin.

### How

This was discussed briefly internally and it was agreed that creating this feature request is fine. Although, it looks like there are no plans in the near future to implement this because it's quite challenging to identify which CSS is unused in the era of JS frameworks. Since there are many ways this could go wrong, it's not certain that this can be done now. In summary, since the team thinks it could be revisited in the future, this feature request is here for tracking purposes.

Start with the linked WordPress support thread and the Jetpack Boost plugin context; the issue names no files, tests, or entry points. Research how unused CSS is identified alongside JavaScript frameworks, then establish safe detection criteria and tests that show styles are not broken.

Written by the indexing model from the issue text.