Perhaps you've discovered (because you are talented indeed and would find something like this) that your XHTML and CSS validates, yet somehow something is still wrong with your layout. Or maybe, you're using some special JavaScripts to handle certain aspects or features of your theme. W3C's XHTML and CSS tools won't validate JavaScript. If you find yourself in this situation, you're going to have to dig a little deeper to get to the root of the problem and/or make sure all aspects (such as JavaScripts) of your theme's files are valid.
You can use Firefox's JavaScript/Error Console (called the JavaScript Console in 1.x and Error Console in 2.x) to debug and validate any JavaScripts your theme is using. Go to Tools | Error Console in your browser to activate it; you can also activate it by typing JavaScript: into your address bar and hitting Enter on your keyboard.
You will be pleasantly surprised to find out that the console will also spit...