I use the TopSite 3.1 style editor. It helps me figure out what attributes work with what HTML tags, and even test them. I would type a tag like "div" and open a curly brace, and it would close the brace and highlight all the attributes i could use, with a chosen level of CSS.

Anyway, it worked pretty well and it saves me a lot of guessing, but I thinks its like 10 years old. I like being backward compatible, but is there anything newer I should know about that's free that might let me see newer options and attributes? Maybe an on-line style editor that's kept up to date somewhere on the W3C schools site?