aakk9999 - 10:40 pm on Jun 17, 2011 (gmt 0)

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.

Yes, it is kind of, but a bit more complicated. With X-Robots you would usually return this for a certain resource type (e.g. all .doc resources or similar), and the number of valid combinations is limited. With an etag, you can go for an algorhitm that is then codded once and applied in many responses.

With canonical in HTTP headers for a pdf document for example - every pdf document would have to have somewhere an information on its canonical resource (e.g. on its canonical HTML page). So somewhere a table of pdf documents (or other resources) with associated canonical info for each, to be returned in HTTP headers.

Perhaps CMS systems like Wordpress will develop plugin for it.

I am wondering what would happen if HTTP Headers return one canonical, and there is also a canonical link element in the head section of the document - which one takes the preference?

<added>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 ?</added>