Search engines have to be able to understand and read the HTML of web pages so they can rank them in the SERPs and match them against search phrases. If the code includes errors, the search engines might not be able to read the page and all that well crafted content and link building can go to waste.

This is particularly important for dynamic database driven sites like blogs and directories, where the same error can be repeated on hundreds of pages.

These days the search engines are can deal with quite a few validation errors, but is it worth the risk? The code of his web pages is one thing a webmaster has complete control over, so it is worth making sure they are error free and as well optimised as possible.

The W3C HTML Validator is the place to go to check a page. Popular HTML editors such as Dreamweaver usually have built in validation tools and you can also find some good add-ons for FireFox that do the job.