The first thing to realise about writing for the web is that people tend to skim-read pages, looking for the salient facts. Therefore it is best to write in an inverted pyramid style, with the most important information and a summary of the article first. Another important guideline is to avoid information pollution (the inclusion of redundant information, like "don't use your hairdryer underwater").
Since people only skim-read on the web, it's a good idea to break your page up with headings, bullet points, and the use of bold to emphasise key points.
Stick to the main topic of the page (don't digress) and include only one idea per paragraph.
About.com has ten guidelines for web writing.