For security purposes, we have to limit certain categories of HTML tags that are very commonly used in media embedding. (Allowing all uses of those tags would provide a vector by which someone malicious could gain limited access to your account or, in the worst case scenario, to others' accounts.) Our fix for that is to place certain sites and their embedding HTML on the explicit allow list and deny any use of those HTML tags that aren't specifically pre-allowed. If there's a particular site you'd like to embed media from and it doesn't work, open a support request with some sample embed code provided by the site, and we can add it to the allow list for you. (Hang on until after the code push, though, since this push involves adding a bunch more sites to the allow list!)