HTML Markup Validation
In terms of SEO (search engine optimization) there are many, many… Factors in getting your website good ranking with search engines. One very important aspect involves how valid your HTML or XHTML code displays across browsers.
Depending on the size of your site and how messy your code is, it could take quite some time getting it all validated. But it is definitely something to look into.
Here is an awesome site that will help you fix your sites errors and problem areas. http://validator.w3.org
It gives you the option to select whether your website is strict XHTML, Transitional XHTML, and so on, or you can use the auto feature. This is a very powerful tool, not only for getting your site optimized for search engines, but it helps ensure your site is being displayed the same across multiple browsers.