Ligatures are missing :( Look at the IE5 screenshot. The second last line has the word "finibus" and notice how the "f" and the "i" seem to be combined together. That's a ligature. In contrast in the body text of the website I don't see any. The word "file" in the paragraph right above the screenshot is missing that ligature. Kerning is, well, also wrong. So wrong that I literally trouble reading the body text, but I…
Yep, we deliberately didn't convert the full set of glyphs from each of the fonts we support, mostly just because it was quite laborious (not to mention CPU time-consuming) to generate our relatively small range of weights, sizes, and font families as-is, so missing ligatures are a known issue. If it's really illegible, i wonder if there isn't some other rendering bug happening – we admittedly didn't do much testing…
For the kerning, ok, that's more work...