Let them make notes and give you feedback. Critiques are often hard to take, but the best way to make sure that your text is working is to find reviewers who will be honest with you. Another technique is to read aloud what you've written. If something doesn't sound right when read aloud, chances are it won't scan well to your reader, either. Make your text clear, concise, and to the point.
Most Internet users are well educated.
Misspelled words and incorrect usage can make you look like you don't care. Some people have Sparcstations and T1 connections, and some people have Mac classics on 2400 bps modems. You can't predict where your web presentation will end up, so you have to anticipate the limitations of the technology on the other end. Remember that the least common denominator is still text. Text is fast and can be displayed on anything.