Does your current website conform to standards? Yes, there are standards that have been set by the World Wide Web Consortium (W3C) on how web sites should be written. Following these standards ensures that your site is readable on all platforms, by future browsers, on mobile devices, and by the visually impaired.
Some search engines check the HTML or XHTML code of your website in search of relevant content. If they find invalid HTML or XHTML code – meaning code that is not following the official rules, you might be removed from indexes. If there is an error on your web page code, spiders will stop searching your whole website, and ths could very well be the end of your optimization efforts.