Learning HTML syntax is the simple part. The tedious part is learning which tags already exist, and which tags goes in which other tag (and which attributes they may (not) contain).
For that, always, ALWAYS go to the official HTML spec: WHATWG HTML.
HTML has not been maintained by WC3 (though they still do maintain CSS) and has been a "living standard" since HTML5 (2009-ish, IIRC).
I've read through the entire spec (using TTS, because only reading is boring) and learned a TON, because writing React straight out of the gate, without learning the HTML fundamentals first is a HUUUUGE pain.
/rant