mrguy - 1:58 pm on Feb 16, 2011 (gmt 0)
In some cases where the HTML error prevents the site from being crawled or viewed correctly, then yes it will perform worse than a site that is valid.
However there are many non valid sites that view and are crawled just fine that out perform sites that are valid because they are better optimized than the valid sites.
It all depends on just what the errors are with the site.
Also, Wordpress works just fine for sites as I have many that are #1 and have been #1 for years beating other sites that are validated.