The text rendering in Qt 5.0.2 and Qt 5.1 (current git) is of poor quality in Qt Assistant. Specifically, the monospaced font used for code examples examples very light and some glyphs are not even visible. Referring to the attached screen shot, note that in "SOURCES declaration" the tops the characters in SOURCES are missing. Even worse, in the code block on the middle of the page, "int main (argc, char *argv[])" the open-paren is invisible, as is the [].

My Qt installation is built without webkit, so I think that Assistant is using one of the rich text widgets.