GRDDL, a mechanism for putting RDF data in XML/XHTML documents, is specified mostly at the XPath data model level. Some GRDDL software goes beyond XML and supports HTML as she are spoke, aka tag soup. HTML 5 is intended to standardize the connection between tag soup and XPath. The tidy use case for GRDDL anticipates that using HTML 5 concrete syntax rather than XHTML 1.x concrete syntax involves no changes at the XPath level.

But in GRDDL and HTML5, Ian Hickson, editor of HTML 5, advocates dropping the profile attribute of the HTML head element in favor of rel="profile" or some such. I dropped by the #microformats channel to think out loud about this stuff, and Tantek said similarly, "we may solve this with rel="profile" anyway." The rel-profile topic in the microformats wiki shows the idea goes pretty far back.

Possibilities I see include:

• GRDDL implementations add support for rel="profile" along with HTML 5 concrete syntax.
• GRDDL implementors don't change their code, so people who want to use GRDDL with HTML 5 features such as <video> stick to XML-wf-happy HTML 5 syntax and they use the head/@profile attribute anyway, despite what the HTML 5 spec says.
• People who want to use GRDDL stick to XHTML 1.x.
• People who want to put data in their HTML documents use RDFa.

I don't particularly care for the rel="profile" design, but one should choose ones battles and I'm not inclined to choose this one. I'm content for the market to choose.