Why isn't it? I'd say as a markup language, it touches on a lot of computer science topics: data structures, semantics, abstraction, etc. Also, I think it's a great entry point that could lead to an interest more "serious" CS topics.
http://blog.acthompson.net/2013/03/html-and-computer-science...
http://supercomputerscience.blogspot.com/2012/07/html-is-not...