Installation guide for PhpStorm 2019.3.1 to minify CSS files automatically after editing using the command line interface for CSSO as a file watcher. You could also use yuicompressor instead, but version 2.4.8 chokes on CSS @keyframes rules.
Update 02.02.2018: As noted by Tom this now only works when the attribute multiple is set. Have you ever wanted to set the background color of a selected OPTION element of a drop down ? Unfortunately, setting the CSS background-color property does not work for (most/all?) browsers, but there is a workaround using the background-image […]