Why validate?

Remember the internet grew very fast (sometimes too fast) so very often its technologies were developed by different groups, at the same time, without a real standard control. This is what happened with HTML. People around the world started writing their web sites using different "standards" causing millions of HTML coded sites to look different depending on the web browser used (and you don't want that to happen to your site). Also, pages containing inconsistent HMLT code are more difficult to be indexed by search engines (and that is something you don't want either).

Today, the internet gods (read the World Wide Web Consortium, or W3C) have finally come to an agreement on what the standards to write an HTML page should be, they called this standard: XHTML 1.0.

HTML stands for Hypertext Markup Language while XHTML stands for Extensible Hypertext Markup Language.

Summarizing, XHTML is nothing but a well formed HTML code. XHTML coded pages should display exactly the same regardless of what web browser you are using (more compatible), and they also should be easier for search engines to read and index.