Although XML validation is by no means a mandatory requirement for search engine optimization, sites that are XHTML compliant have certain advantages over sites that are not XHTML compliant. To understand the importance of XHTML compliance it is important to first understand what is XHTML? XHTML was designed to act as an improvement over HTML, with devices other than computers accessing the internet, it was important to develop a language that could ensure pages and sites were viewable across devices. In a nutshell, an XHTML compliant website is cross device compatible and handles errors more robustly.
Getting back to the topic, what does XHTML compliance have to do with SEO? XHTML compliance ensures easy accessibility by Google bots. Pages that are XHTML compliant are usually indexed much faster by Google and other search engines. Since XHTML code is also easier to ‘understand’, Google bots and other search engine bots have no trouble in accessing and downloading content on a regular basis. Apart from XHTML compliance, newer websites are also attempting to become HTML 4.01 compliant. News is, sites that are HMTL 4.01 compliant stand a better chance of frequent indexing and caching of pages. As it stands today, Google takes a very long time to index pages, pages that are XHTML compliant are usually given the option to be indexed faster by the Google Webmasters tool.
HTML 4.01 and XTHML were suffering from a little bit of an identity crisis; both were created and then left to fend for themselves. Google in particular has given the concepts of XHTML and HTML 4.01 a fresh lease of life by encouraging people to make their websites and web pages XHTML and HTML 4.01 compliant. XHTML and HTML 4.01 compliance alone cannot ensure better search engine ranking, there are numerous websites that are neither XHTML nor HTML 4.01 compliant but are ranking well with search engines.
What compliance with W3C standards ensures is that websites are ready for future changes in SEO and it also ensures that websites are easily accessible by various search engines. What is equally important to note is that there is a definite correlation between XHTML compliance and the rate at which sites are indexed and cached by Google. In addition, making pages XHTML compliant is also believed to protect pages from the dreaded Google 950 penalty. XHTML and HTML 4.01 compliant sites are also supposed to be protected against problems like the Google Supplement.
In conclusion, there is considerable speculation if XHTML and HMTL 4.01 compliance is important or not, but as is the case with most search engine optimization principles, there is no way to know for sure if a site that is XHTML compliant performs better than a site that is not XHTML compliant or not. Search engine optimization is not governed by a single factor like XTHML compliance or effective link building. It is therefore important to keep an eye out for various factors that might help improve the ranking of a website, XHTML and HTML 4.01 compliance is just one such thing.