Federal Plain Language Guidelines
Although oriented towards helping U.S. government employees write clear regulations, the Federal Plain Language Guidelines offers great advice for any nonfiction writer. It includes a section on writing content for web sites.
Here are some highlights.
“Address one person, not a group. Remember that even though your document may affect a thousand or a million people, you are speaking to the one person who is reading it. When your writing reflects this… [it] has a greater impact.”