However the greatest advantage that you can get from a site with clean HTML is its intrinsic search engine friendliness.
SEO (Search Engine Optimization) is widely regarded as one of the best ways to improve the visibility of your website and enhance the flow of traffic to it, both qualitatively and quantitatively. And when you have a clean site with well written HTML, consider half the battle won.
Webmasters and SEO experts around the world equivocally support the cause of clean HTML. The simple reason behind this is that search engine spiders are better able to crawl a site with clean HTML than one where there are unnecessary lines of coding, missing tags, badly-defined attributes and so on.
Here is an excellent 20 step guide to ensure that your site has clean HTML for better search engine friendliness and user experience.