Dream scenario: 1/ run against generated output via url submission, 2/ receive optimized output, 3/ point at source generator tree, 4/ receive suggested source diffs
Lucid dream scenario: point at repository, receive optimized source diff suggestions on a per-file basis
most of the removed css isnt necessarily useless globally, it's just useless in context. eg a shared bootstrap css grid will be pruned differently on different pages, but there's no single "suggestion" that can be used to modify that shared initial grid.
for css that's leftover by accident that should actually be removed from the source, i agree. but how will you discern the first scenario from the second?
Edit, also pls send me a pony. Optimzations have a few audiences, the primaries of which are the visitor and the developer. Source tree prunes ("developer audience") not addressed in my off-the-cuff pipe dream above