For the current document, the validator interprets strings like
<FOO /> according
to legacy rules that break the expectations of most authors and thus cause confusing warnings and error messages from the validator. This interpretation is triggered by HTML 4 documents or other SGML-based HTML documents. To avoid the messages, simply remove
the "/" character in such contexts. NB: If you expect <FOO />
to be interpreted as an XML-compatible "self-closing" tag, then you need to use XHTML or HTML5.