CSS blocks rendering, meaning that while CSS is downloading and the CSSOM is being built no rendering is done on the screen. However the HTML is still being parsed and the DOM is still being constructed.
Imagine that you first include a CSS file and then a JS file. The parser first encounters the CSS file and starts downloading it. It then discovers the JS file, stops parsing to fetch to fetch and execute it. However if the CSSOM is not yet ready than JS will halt execution and wait on the CSS.
Only when the previously encountered CSS file has been downloaded and the CSSOM created does the JS begin to execute. All this time parsing has been paused.
HTML | Parsing/Building the DOM | | Parsing is paused to allow JS to execute | | Parsing continued | CSS | Fetching the CSS | | Building the CSSOM | JS | Fetching JS | | Blocked | | JS executes |