We'll create fresh WordPress site with AddThis xmlns installed. You have 20 minutes to test the plugin after that site we'll be deleted.
This plugin adds the xmlns:addthis=”http://www.addthis.com/help/api-spec” entry to the HTML tag. This head entry is required to make it Valid XHTML. More info at http://www.addthis.com/help/client-api#valid-xhtml