When using either the HTML4 writer or HTML5 writer, if any <meta> tags are added to the document with a name attribute that starts with "dcterms.", then the writer (in the depart_document() method of both docutils.writers._html_base.HTMLTranslator and, in the current Subversion snapshot, docutils.writers.html5_polyglot.HTMLTranslator) will add the following tag to the document <head>:
<link rel="schema.dcterms" href="http://purl.org/dc/terms/">
However, this link is not properly closed (It should end with />), and so any document containing it will not be valid XML.
Fixed in r8528.
Thank you for reporting.
Fixed in Docutils 0.17.
Thanks again for the report.