The canonical link element is a great tool for dealing with duplicate content issues. That's good for website owners, particularly in certain industries where duplicate content can be a real pain, such as ecommerce.
But up until now, the canonical link element didn't have much utility in addressing duplicate content issues across multiple domains. But Google is changing that thanks to its newfound support for cross-domain canonical link elements.
Using the new cross-domain canonicals is easy. Just use rel="canonical" elements as you normally would using a full URL, domain included. Example:
<link rel="canonical" href="http://www.yourfirstsite.com/about-us.html" />
It should be noted that Google looks at the canonical link element "as a hint and not an absolute directive" but will "try to follow it where possible". As far as I can tell, Bing and Yahoo have not announced support for cross-domain canonicals, so depending on how other search engines are in your traffic mix, it may still make sense to address cross-domain duplicate content issues with existing techniques, such as 301 redirects.
Of course, those are not always viable, which is why Google's announcement is welcome news. As Google software engineer Greg Grothaus points out on his blog, a canonical is very useful for accomplishing certain things. A few he mentions:
- Moving a website to a new domain but not having access to modify headers to add a 301 redirect.
- Keeping identical or similar landing pages on different domains from all appearing in Google.
- Allowing partners to whom you syndicate content to pass the "link juice" back to the original source -- your website.
For more information on canonicals, be sure to check out Google's updated help page.