HTML Tags You Can Use on GitHub
Wherever HTML is rendered on GitHub (gists, README files in repos, comments on issues and pull requests, ...) you can use any of the HTML elements that GitHub Flavored Markdown (GFM) provides syntactic sugar for. You can either use the syntactic sugar that GFM (or other GitHub-supported markup language you're using) provides or, since Markdown can contain raw HTML, you can enter the HTML tags manually.
But GitHub also allows you to use a few HTML elements beyond what Markdown provides by entering the tags manually, and some of them are styled with CSS. Most raw HTML tags get stripped before rendering the HTML. Those tags that can be generated by GFM syntactic sugar, plus a few more, are whitelisted. These aren't documented anywhere that I can find. Here's what I've discovered so far: