I agree HTML shouldn't probably be considered a programming language. It's a container-based markup language. A programming language in the traditional sense, would force HTML to be able to alter the behavior of the computer. Such is not the case.
However HTML is no toy as it was suggested either. Markups have become increasingly more complex over the years. HTML and XHTML have high standards and best practices, they do take years to master and force the "programmer" to think logically and to structure their "code".
They are probably the closest you can get to a programming language without becoming one