I’ve been working my way through HTML, XHTML & CSS for Dummies which is all very exciting. It’s early stages at the moment, so most of what it’s told me so far isn’t new (apart from the fact that in XTML, empty elements must end with a slash before the closing greater than symbol and there should be a space before the slash like this <tag />).
Anyway, the book has a habit of mentioning useful links without really explaining them or drawing any attention to them. One such link is http://validator.w3.org. This is a free markup validation service which will highlight any errors or issues with your markup. (You can get it to check any webpage and it’s actually quite fun putting some of your favourite sites in and seeing how many errors they come back with!).