actually, there is a reason. because in IE6 it breaks depending on context. if you have an incorrect content header and don't prefix <a href="http://" rel="nofollow">http://</a> on your url, then having an html 5 doctype makes your page try to "download" instead of display.<p>Whereas having a doctype that matches the tags you actually use, (xhtml or html4) will work fine without the <a href="http://" rel="nofollow">http://</a> prefix. As most people don't use that prefix when typing your url, this can have a drastic effect on your page views.