Benjamin Francis <bfran...@mozilla.com>, 2015-06-03 19:42 +0100: ... > Open Graph is quite primitive in comparison to other formats in terms of > what can be expressed (and it's not clear to me whether it validates as > either valid HTML5 or valid RDFa)
It’s not valid HTML(5), because the HTML spec doesn’t allow the meta element to have a `property` attribute. But it is valid HTML+RDFa (Lite), because the HTML+RDFA spec extends the HTML spec to add the `property` attribute to the meta element (and as far as I understand it least, Open Graph markup entirely consists just of using meta[property] and meta[content]). As far as how HTML-checking tools deal with that markup, the shared backend code for validator.nu and the W3C Nu HTML Checker at the HTML5 facet of the legacy W3C validator by default doesn’t report errors for meta[property] (or for any other RDFa Lite attributes in HTML documents). In other words, it essentially treats it (and all other RDFa Lite markup) as being valid. We made that behavior the default specifically because we were getting enough bug reports from users about the checker reporting errors for Web documents containing Open Graph meta[property] that we decided it’d waste less user time if we were to allow it rather than emitting errors for it. And that sorta necessarily meant also allowing all other RFDa Lite attributes, because there is no actual Open Graph spec, and the HTML+RDFa spec is the only spec that does actually define a meta[property], and RDFa Lite was the smallest defined subset of RDFa which would get us that. Beyond that I think we don’t (can’t) do any useful datatype/microsyntax checking of the actual value of meta[property] due to the fact the RDFa specs define it as being allowed to contain CURIEs, while at the same defining CURIES in such a way that any arbitrary string can be a valid CURIE. So in practice authors can put anything they want into meta[property] without the checker reporting any errors. —Mike -- Michael[tm] Smith https://people.w3.org/mike
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