To close or not to close – Void HTML elements
HTML loves its special cases.  XML is overly complex, but at least your editor doesn't need to know anything special about what document type you're writing in order to indent it properly.  Throw in HTML's special cases, and now it needs to know that <br> is different from <foo>.<p>I guess since HTML is so common it doesn't really matter, but really?  We need 5 differnt types of markup, when one would have been fine?<p><a href="https://xkcd.com/927/" rel="nofollow">https://xkcd.com/927/</a>