HTML5 Reference by MDN
You don't need to know every HTML5 element. Here you can find and this is excellent reference all the time.
HTML5 Reference by W3Schools
This HTML tutorial site contains hundreds of HTML examples. With online HTML editor, you can edit the HTML using editor.
HTML Entity Code by CSS-Tricks
Super useful reference, gives you HTML entitiy name, numeric code, hex code and ISO code.
HTML Cheat Sheet
Shotcut Code commands for a very handy printable HTML5 cheat sheet that can save you a ton of time.