I have a fully valid website, save one error:
<meta property="og:locale" />
The error is because property is not a valid attribute for the meta tag. The site is a Joomla site with an XHTML 1.0 Transitional doctype. I found this thread from two years ago, but for some reason the w3c validator did not recognize the suggested doctype fix:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/html401-rdfa-1.dtd">
I'm not sure if that it because my current site is in XHTML1.0 transitional and the fix is for HTML4.01 transitional? (Also, a whole bunch of new errors were found with the 4.01 doctype, so I'm not really trying to change doctypes.)

Is there a known solution for this validation error that doesn't involve changing doctypes? Or, one that works with the site's current doctype (XHTML1.0 transitional)?