W3c is the human body that tries to standardize this stuff, they compose the rules, then programmers compose their rendering engine to interpret those rules. Here is some code that I a short while ago wrote. I believe that it provides a essential explanation of mixing course/ID names with pseudoclasses. https://bookmarkproduct.com/story19465658/prestashop-popup-youtube-video-colorbox-on-all-pages-can-be-fun-for-anyone