WiseGeek has some best-practices when writing on the web:

• Use short and concise sentences. It’s harder to read from a screen than from a paper and readers tend to slow down.
• Keep paragraphs short. Paragraphs are often around 50 words instead of 250 for conventional writing. That’s about 7 or 8 sentences.
• Don’t use “click here.” Readers don’t like the words “click here” but prefer the link to be words that give the reader information.
• Keep documents short. Documents longer than 10 pages should be a download.
• Avoid parentheses.
• Spell properly and be careful about spelling errors that are homonyms (e.g. using righting instead of writing)
• Define acronyms when they first appear.