PurifyCSS
Remove unused CSS. Also works with single-page apps
... and built into your build process using tools like Grunt, Gulp or Webpack. However if you're using a different development stack or building your sites with a CMS like Wordpress, you can use the online tool to clean up your stylesheet. Especially if you are using CSS frameworks like Bootstrap or a multi-purpose Wordpress theme, chances are you are actually using less than 20% of the defined CSS rules.