RDFa introduced a property attribute for the <meta> element, and the W3C even recommends this as an extension to HTML5. Facebook's Open Graph protocol, for example, uses the RDFa property attribute like this (example from the Open Graph site):
<html prefix="og: http://ogp.me/ns#">
<head>
<title>The Rock (1996)</title>
<meta property="og:title" content="The Rock" />
<meta property="og:type" content="video.movie" />
<meta property="og:url" content="http://www.imdb.com/title/tt0117500/" />
<meta property="og:image" content="http://ia.media-imdb.com/images/rock.jpg" />
…
However the HTML5 specification seems to prohibit this usage. I'm not talking about whether it allows the property attribute; I'm referring to its explicit prohibition of the content attribute without a name attribute for the <meta> element:
If either
nameorhttp-equivis specified, then thecontentattribute must also be specified. Otherwise, it must be omitted.
Isn't this in direct conflict with current RDFa usage such as in Open Graph? The HTML5 specification seems to require the presence of a name attribute as well here.
The W3C Recommendation "HTML+RDFa 1.1" extends the HTML spec (you can find all extensions in a W3C Note).
This extension changes HTML’s conformance requirements for the
metaelement:So, these two HTML+RDFa elements are valid:
(The other two
metaelements are invalid, because they have URL values, for which thelinkelement must be used instead.)