http://www.w3schools.com/
http://www.w3schools.com/html/default.asp

Not only can you learn HTML, but there are several other languages that actually integrate with HTML there too. You can try the examples in realtime on the site itself, so playing around and tweaking is instant.

Plus, it is very important to have valid code and not use incorrect syntax or deprecated HTML. CSS will come into play too as there is very little to HTML and all the style remains in HTML.