Step one: Making your website valid.
First things first: let's find the bad guys errors.
The lean, mean cleaning machine
Type in your web address below and press enter to see how your site is in terms of errors (by the W3C's Markup Validation Service- fancy jargon for a service that allows you to check your website for whatever scandalous errors you may have)
Fix the errors by looking at what they highlight, researching what they mean, and checking again. There are countless resources online that will help you in reducing any errors you may have. It won't involve stripping your website apart and building from scratch, and often the fixing of a single error will reduce the number of all errors by a few, since a lot of HTML and CSS depend on each other.
You can also check out Keynote's Net Mechanic, which will provide you, on top of identifying any errors, with tips on how to fix things.
Keep in mind that there are some things that are unfortunately beyond your control, especially if you use an external service. For example, if you use Blogger or Xanga or some other online publishing service, chances are that they have things they include that you really don't have access to. Sometimes these areas are, unfortunately, not valid.