Actually the help system can be used as a simple web browser. Help text can contain links to websites; You can go to any external site (if you are connected to the net) by selecting the File radiobutton in the Help toolbar, typing in the full url name in the entry, and pressing Return.
The HTML class that can be based on the compiled tkhtml widget by D. Richard Hipp or the Tcl-only html library by Stephen Uhler. The html library one only understands HTML 2.0 and is slower than the compiled one. It is however distributed together with ClassyTk. Tkhtml supports HTML 3.2. and a lot of neat things like tables. It has a different copyright (LGPL) from ClassyTk and is distributed seperately. ClassyTk tries to load tkhtml by doing a "package require Tkhtml". If this does not load the tkhtml widget, it reverts to the tcl-only version. The native interface of tkhtml is described here.