This resource lists some of the most common problems encountered when writing HTML. If none of these answers solves the trouble you’re experiencing, try running your page through an online validator, such as the one offered by the W3C at validator.w3.org. A service like this tests your page against the HTML specifications and displays a list of errors that makes it easy to locate problems.
Or, you could use Dave Raggett’s HTML Tidy program (available for download at http://sourceforge.net/projects/tidy/). This handy program actually attempts to fix any problems on your page, if possible. It can also display a list of errors to alert you to things you can look for in the future.
Also, consider downloading a few ...