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www.w3schools.com
Their tags reference is handy.
Also, the specifications:
https://html.spec.whatwg.org/multipage/?
If you're looking for more than just html5/xhtml and looking more broadly to web developers, I also like:
https://css-tricks.com/almanac/
Some of it is a bit outdated, but otherwise it's pretty great.
This seems to get a fair bit of attention:
https://developer.mozilla.org/en-US/
but I find it hard to grok sometimes.