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Based on your feedback, we’re happy to announce that Google web search now supports link rel="canonical" relationships specified in HTTP headers as per the syntax described in section 5 of IETF RFC 5988. Webmasters can use rel="canonical" HTTP headers to signal the canonical URL for both HTML documents and other types of content such as PDF files.
It's kind of parallel to the X-Robots directive. I don't think the average webmaster even knows about that, either. Or etags, or a host of other technology tools that can be very useful.
I don't recall ever having had duplicate content problems with non-HTML doctypes.
[edited by: phranque at 2:27 am (utc) on Jun 18, 2011]
I am wondering if this works cross-domain? Because it could be a heaven for hackers - it is bad enough searching for canonical insertion in your html via view source, but now it seems we would have to inspect headers too ?
I'm having nerdgasms.