I just notice when I played a bit with hfyview.el that there is a bug
in hfy-fontify-buffer. Or at least I believe it is there.

The problem seems to be that a substring is copied with all text
properties to the new buffer. Some of them will prevent htmlfontify
from working correctly. I think the call to copy-buffer should be
replaced with something that does the same thing but removes some of
the text properties.

This is what I just did in hfyview.el, but that is the wrong place to
do it. See line 512 in the attached hfyview.el for some suspicious
text properties (I am not sure how they work so I took them all.
Beside that I saw that font-sticky and rear-nonsticky is not described
on (info "(elisp) Special Properties").)

I found the problem when I did like this:

 load hfyview.el
 M-x hfyview-frame-mode
 M-x
 Push <apps>

This will work now, but that is due to that the call to propertize on
line 515 in hfyview.el removes some text properties from the variable
str before the insertion in the buffer. (I am not sure which ones that
helped here.)

If you do not remove these text properties then the content of the
minibuffer will not be shown in the htmlfontify output.

I have attached hfyview.el