I had an intro web development workshop. First session was only about HTML. I learnt 3 things I did not know about HTML:

1. Footers don't necessarily have to appear at the end of a section, though they usually do [0]

2. A head element's start tag can be omitted if the element is empty, or if the first thing inside the head element is an element. A head element's end tag can be omitted if the head element is not immediately followed by ASCII whitespace or a comment [1]

3. Self-closing tags (<tag />) do not exist in HTML [2]

Please let me know if you know something else.

[0], [1] whatwg.org [2] developer.mozilla.org