You'll always want to validate your XHTML first. This is just as well because W3C's CSS validator won't even look at your CSS if your XHTML isn't valid.

Go to http://validator.w3.org/ and if your file is on a server, you can just enter in the URL address to it. If you're working locally, from your browser, you'll need to choose Save Page As and save an HTML file of your theme's WordPress output and upload that full HTML file output to the validator using the upload field provided.

In our example above, you can see that we have a typo in one of our divs (looks like an odd s got in there somehow), and we have an image tag that doesn't have the proper closing (/) in it. Wherever possible, you'll note that the validator tries to tell us how to fix the error. Whenever a recommendation is made, go ahead and implement it.

We'll need to fix those two errors and run the validation again to make sure we're now validating. Don't just think you can fix the errors listed and move...