[andreasvc]

Mathematical formulas are a big reason; wikipedia uses images which look passable unless you start zooming in. Another reason is that with HTML you're never sure if you'll end up with the fonts as they were intended. Lastly, because HTML is not a page-oriented markup format, the presentation in general can differ from system to system; e.g., you can't say "on line 2 of page 3".

[scott_s]

You may not have seen MathJax: http://www.mathjax.org/ The main drawback is that I noticed a delay before it fills in the correct math notation. But the results are quite impressive.