W3Schools
https://www.w3schools.com/html/
W3Schools is a web developers site, with tutorials and references on web development languages such as HTML, CSS, JavaScript, PHP, SQL, Python, jQuery, Java, W3.CSS, and Bootstrap, covering most aspects of web programming.
Webmonkey Tutorials
http://www.webmonkey.com/tutorials/
“Webmonkey, the web developer's resource, offers free learning guides for a variety of programming and coding needs.”
HTML Quick Reference
http://www.techdocs.ku.edu/index.shtml?html
This is an extremely useful quick reference guide for those not willing to slog through an entire tutorial or primer to get the information they've forgotten.
World Wide Web Consortium (W3C)
http://www.w3.org/
"The World Wide Web Consortium (W3C) is an international consortium where Member organizations, a full-time staff, and the public work together to develop Web standards. W3C primarily pursues its mission through the creation of Web standards and guidelines designed to ensure long-term growth for the Web. Over 400 organizations are Members of the Consortium. W3C is jointly run by the MIT Computer Science and Artificial Intelligence Laboratory (MIT CSAIL) in the USA, the European Research Consortium for Informatics and Mathematics (ERCIM) headquartered in France, Keio University in Japan, and has additional Offices worldwide."
The Web Design Group
http://www.htmlhelp.com/
"The Web Design Group was founded to promote the creation of non-browser specific, non-resolution specific, creative and informative sites that are accessible to all users worldwide. Such sites need not be boring and non-graphical!" Contains excellent HTML Reference materials, a good collection of HTML-related links, and an inline HTMLHelp BBS where you can ask questions.
HTML Goodies: The Ultimate HTML Resource
http://wdvl.internet.com/
"A comprehensive illustrated encyclopedia of web technology, The Web Developer's Virtual Library is a well-organised goldmine of tutorials, examples, and links to great resources. It's for webmasters and Internet developers who are creating web sites with HTML, CGI, Java, JavaScript, graphics, VRML, multimedia, animation, etc."