Answer 1

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Some of it is a bit outdated, but otherwise it's pretty great.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.