He will get context on what HTML is, and how it interacts with the browser. It's much easier to internalize knowledge with this website than it is elsewhere.
Keep the link as a reference but it also provides some good quizzes to learn and practice basic concepts. 8 years ago that's where I started learning HTML/CSS/PHP.
http://htmldog.com/guides/html/beginner/gettingstarted/
He will get context on what HTML is, and how it interacts with the browser. It's much easier to internalize knowledge with this website than it is elsewhere.