This stuff is relevant for sites that allow users to upload CSS to be used by other people. If I can make a subreddit or a social media page on a site and uploa

This stuff is relevant for sites that allow users to upload CSS to be used by other people. If I can make a subreddit or a social media page on a site and upload custom CSS for it, then I can make the CSS trigger requests to my own personal server on certain events and track the people who visit my subreddit/page. (Reddit adds certain restrictions to CSS that can be uploaded to it to defend against this.)