Animista is a place where you can play with a collection of pre-made CSS animations, tweak them and get only those you will actually use.
When you are building a website with a css framework like Bootstrap, you will only use a part of the framework and unused css styles will be included.
Purgecss removes unused selectors from your css, resulting in smaller files.
"With PurgeCSS, you'll have a hard time generating CSS that isn't under 10kb minified and gzipped." - Tailwindcss docs
Unused CSS finder crawls your website (following all the internal links), analyses all the pages and provides a list of unused css-selectors found in your stylesheets.