What is W3C Compliancy?
W3C is a worldwide agency that develops the standards for website coding. Major search engines check these standards against your website, if you have errors within your code, this will count against your ranking. A lot of add-ons such as flash and some javascripts create errors within your site, unfortunately this is inevitable. As long as the errors are kept to a minimum due to add-ons, there shouldn’t be much of an issue.
