HTML5 doctype with RDFa I'm using Omega 3, and have a redundant HTML doctype header line.  

How do I correct this so I have my HTML5 &lt;!DOCTYPE html&gt; line, and specify RDFa? 

Currently, the first two lines of my pages are:

&lt;DOCTYPE html PUBLIC "-//W3C//DTD HTML+RDFa 1.1//EN"&gt;
&lt;html lang="en" dir="ltr" version="HTML+RDFa 1.1"...&gt;
Omega4 supports 

&lt;!DOCTYPE html&gt;
&lt;html lang="&lt;?php print $language-&gt;language; ?&gt;" dir="&lt;?php print $language-&gt;dir; ?&gt;"&lt;?php print $rdf_namespaces; ?&gt;&gt;


where you don't have to manually define RDFa.

Beside Omega4 I normally do (usually I use Zen, but you can do this in other themes too):

&lt;!DOCTYPE html&gt;
&lt;html lang="en" prefix="dc: http://purl.org/dc/elements/1.1/"&gt; 


and define property="http://purl.org/dc/terms/TERMS wherever I use RDFa

Dublincore Metadata Initiative has detailed information about the terms

Drupal7 core has RDF support via rdf_namespaces. The hook implementation is: 

function hook_rdf_namespaces() {
  return array(
    'content' =&gt; 'http://purl.org/rss/1.0/modules/content/',
    'dc' =&gt; 'http://purl.org/dc/terms/',
    'foaf' =&gt; 'http://xmlns.com/foaf/0.1/',
    'og' =&gt; 'http://ogp.me/ns#',
    'rdfs' =&gt; 'http://www.w3.org/2000/01/rdf-schema#',
    'sioc' =&gt; 'http://rdfs.org/sioc/ns#',
    'sioct' =&gt; 'http://rdfs.org/sioc/types#',
    'skos' =&gt; 'http://www.w3.org/2004/02/skos/core#',
    'xsd' =&gt; 'http://www.w3.org/2001/XMLSchema#',
  );
}


so you can simple do &lt;?php print $rdf_namespaces; ?&gt; in your html.tpl.php file where you start &lt;html&gt;.

If you want latest RDF support there is a module for that one too called RDFX.