CSS Junk files in the css folder What are all those junk CSS files in site/files/css with names like css_ZFCUUB6IIfaVQ-erWDr0M0Na1JbxEsU-8zjQrWAtrbU.css? There are hundreds of them.

How do these get generated over time? What do they do?
They aren't junk files, they are aggregated files are generated when you have CSS aggregation turned on. They make your page load times faster, as multiple files are aggregated into a single file, meaning less files have to be downloaded to the browser.