W3Schools index has been a LIFESAVER for me!! I find a neocities page I like, right click and click "view page source", then look through the index to see what each tag means. I'm slowly relearning HTML that way. Sometimes you even find the source of the layout they used (if it's a base or from a specific website) and you can go to that source to find the original.
Since we have already mentioned w3schools (the greatest website for learning how to code), i would to add Mozilla Web Docs into the mix. Its helped me when im writing code with elements i am not familiar with.