Html: Resources
HTML resources.
MDN HTML Docs
The MDN does such a spectacular job in documenting HTML that what I add on this site is minimal. In particular, MDN does not get tied up in worrying about the various specifications and tussles around this, they focus on how HTML works in practice on the current popular browsers and as a web developer, this is exactly what you need. Bookmark the following link and consider it a one-stop shop.
W3C - World Wide Web Consortium
The World Wide Web Consortium (W3C) develops standards and guidelines to help everyone build a web based on the principles of accessibility, internationalization, privacy, and security.