Once the RDF module has been enabled the HTML validator complains about tons of issues:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML+RDFa 1.1//EN">
<html lang="de" dir="ltr"
  xmlns:content="http://purl.org/rss/1.0/modules/content/"
  xmlns:dc="http://purl.org/dc/terms/"
  xmlns:foaf="http://xmlns.com/foaf/0.1/"
  xmlns:og="http://ogp.me/ns#"
  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
  xmlns:sioc="http://rdfs.org/sioc/ns#"
  xmlns:sioct="http://rdfs.org/sioc/types#"
  xmlns:skos="http://www.w3.org/2004/02/skos/core#"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema#">
<head profile="http://www.w3.org/1999/xhtml/vocab">
  <meta charset="utf-8" />

Validator wines about xmlns:* and tons of other issues.

Comments

Title:With RDF module enabled validation failsValidation fails with RDF module enabled

Title:Validation fails with RDF module enabledValidation fails with RDF module enabled, invalid rdf_namespaces added