HTMLteacher

Teaches HTML through interactive lessons that let you try out each new tag and see the results in a browser pane right away. At first, you choose tags from a menu, to eliminate misspellings; then you're free to type them yourself. A built-in HTML checker points out errors. Best of all, you can upload pages to a real website from within the app, with no signup or extra fees required.

Intended for students from grades 4 and up, but suitable for adults, too. (Not for those who already know HTM basics.)