OK, I only now got around to try some HTML files (my old hand-formatted strict HTML/OPF ebooks, and some more recent ones)... How to describe it... err... I guess iRex thinks that HTML is really plain text with some useless tags in it, which you simply ignore... wow...
No inline images, no formatting to speak of, no links to other HTML files, CSS files are unrecognized... I feel like it's 1998 and I'm back on my REB1100... anybody else had better luck?
I get the impression iRex focused on fairly solid PDF support out of the gate and everything else fell on the wayside...
BTW. RTF format is definitely not recognized.