Google is clearly going for some kind of record with product launches this quarter. Today we see them extending support for the rel=canonical tag across two different domains as well as adding sentiment analysis to reviews in Google Local.
I don’t see any situations where a cross domain rel=canonical tag would be the right thing to use but here is what Google has to say:
Q: Do the pages have to be identical?
A: No, but they should be similar. Slight differences are fine.
Q: For technical reasons I can’t include a 1:1 mapping for the URLs on my sites. Can I just point the rel=”canonical” at the homepage of my preferred site?
A: No; this could result in problems. A mapping from old URL to new URL for each URL on the old site is the best way to use rel=”canonical”.
Q: I’m offering my content / product descriptions for syndication. Do my publishers need to use rel=”canonical”?
A: We leave this up to you and your publishers. If the content is similar enough, it might make sense to use rel=”canonical”, if both parties agree.
Q: My server can’t do a 301 (permanent) redirect. Can I use rel=”canonical” to move my site?
A: If it’s at all possible, you should work with your webhost or web server to do a 301 redirect. Keep in mind that we treat rel=”canonical” as a hint, and other search engines may handle it differently. But if a 301 redirect is impossible for some reason, then a rel=”canonical” may work for you. For more information, see our guidelines on moving your site.
Q: Should I use a noindex robots meta tag on pages with a rel=”canonical” link element?
A: No, since those pages would not be equivalent with regards to indexing – one would be allowed while the other would be blocked. Additionally, it’s important that these pages are not disallowed from crawling through a robots.txt file, otherwise search engine crawlers will not be able to discover the rel=”canonical” link element.
Notice how it only works if the content is “similar” just like 301 redirects.