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Validators

W3C HTML Validator: This should be every coder's first port of call when you're working on your HTML. This free online validator will look through your code and pick out every error it finds. It may seem harsh at first, but going along with its suggestions is almost always a good move. If you're transitioning to XHTML this is a great way to keep tabs on your progress. And when you get through with no mistakes (and pick up your congratulatory graphic), it's a great moment.
» validator.w3.org
W3C CSS Validator: This is the same deal as above, but it will look through your style sheets for errors. It can tell you whether you comply with a particular version of CSS (CSS-1 or CSS-2), and give you prioritised feedback on your errors. Again, essential.
» jigsaw.w3.org/css-validator
W3C Link Checker: You don't want to let any broken links slip through the cracks and into your website, so you should give it a good going over every once in a while with a link checker. The W3C have a nice one here.
» validator.w3.org/checklink
Web Design Group Validators: If you find the W3C validators a little too technical, you can get an alternative opinion from these validators (for your » HTML and » CSS). The error messages are much nicer, with friendlier advice offered to solve your problems. The HTML one can validate up to 100 pages at once too.
» www.htmlhelp.com/tools
HTML Tidy: This is an excellent tool, born out of Dave Raggett's fevered mind. It will perform a plethora of adjustments to your HTML code to get it working properly, from fixing up your tag syntax to completely converting your HTML into valid XHTML. A time saver. Bit difficult to get working, mind.
» www.w3.org/People/Raggett/tidy
Accessibility Validator: A free validator that will check your site for browser compatibility and compliance with content accessibility guidelines. The nature of this means that not everything can be automated, so a degree of manual checking is required, but it does point out possible mistakes as well as the ones he can find. A good service.
» webxact.watchfire.com