The fact that text/html derives from text/plain brought back a bug that I fixed in 2002:
"embed katepart and then type a website URL -> loaded into katepart".

The test for mimetype inheritance in changeViewMode came from bug #108542, but that was wrong [the current mimetype shouldn't matter, only what the view supports, otherwise viewing a text/plain file or a C++ file in katepart changes what happens when opening a text/html file later on].

... and in kde4 it should be different anyway with dolphinpart handling all main viewmodes.