According to Wikipedia, typography is
“is the art and technique of setting written subject matter in type using a combination of fonts, font size, line length, leading (line spacing) and individual and overall letter spacing.”
Quite a mouthful. And having written many technical papers and reports as well as writing online, I can personally say that it is no mean feat.
Well, here are some discussions from people much more knowledgeable than me about improving readability for online reading.