- The W3C HTML reference
- The W3C CSS2 reference
- A list of HTML entities
- Tidy, a validator extension for Firefox (this site should pass it …)
- Firebug. The Firefox extension for web developers, it not only performs detailed validation and error reporting; it also allows you to inspect and modify the DOM tree, change CSS properties on the fly, and much more.
- Chris Pederick's Web Developer toolbar is the…other Firefox extension for web developers. The reason why I love it is simple: It allows you to view generated source, when you create HTML on the fly with scripts, which makes it a hell of a lot easier to debug dynamic web pages than merely looking at the static HTML and script source.