Terry Mickelson makes a good point in his article, “Proper HTML Code For SEO” about how even the smallest errors in your HTML can really screw up your rankings.
One thing you can do to help you clean up your code is to use W3C’s HTML Validator to help you find any misspelled, misused, or even extra HTML. After you validate your code, go through it, line by line, just to make sure everything has a closing tag or closing carat (>). If you’re using Dreamweaver 8, you can “close up” parent tags to make sure you don’t have any extra table cell closing tags or to get some of the code out of your way to make it easier to correct. Great tool.
“Internet Explorer is very forgiving of mistakes in code. By building only for IE you are creating for the most error forgiving browser ever. Thinking that a search engine will “browse” your site like Internet Explorer 6 is a dream. If the site won’t display right in Netscape, Firefox, Opera or an old version of IE chances are good the search engines can’t read it either.”