DropCSS

DropCSS is an unused CSS cleaner; it takes your HTML and CSS as input and returns only the used CSS as output. The core is simply some minimal glue between these awesome low-level tools:

• Fast HTML Parser • CSSTree • css-select

The entire logic for DropCSS is this ~60 line file.

It is recommended to also run your CSS through an optimizer like clean-css to group selectors, merge and remove redundant rules, purge unused keyframes, etc. Whether this is done before or after DropCSS is up to you, but since clean-css also minifies, it probably makes sense to run DropCSS first to avoid bouncing [and re-parsing] the output back and forth (optimize & minify -> drop) vs (optimize -> drop -> minify), though this will likely depend on your actual input; profiling is your friend.

A bit more on this project's backstory & discussions in /r/javascript and on Hacker News.