The W3 World Wide Web Consortium publicise specifications for web based content and how it should be structured for consistent representation across different platforms and browsers. W3 also develop guidelines widely regarded as international standards for web accessibility – the web accessibility initiative (WAI).
If web based content conforms to W3 standards for valid HTML, XHTML, CSS and WAI then that content is expected to be accessible to a wider audience. For this reason search engines are likely to add more weighting to ‘valid’ sites rather than sites that do not conform.
Google is beginning to move in the direction of favouring valid sites with ‘accessible search’ – http://labs.google.com/accessible/. This search ranks sites for the search string based on how accessible Google judge that site to be. Although this search is still in a beta â€“ or ‘Google labs’ it is widely expected that such technology will be used in the main Google search.
You should begin to work towards making more content valid and accessible by following specifications and guidleines on the W3 website by using CSS and HTML validators.