If you use Google Webmaster tools, or otherwise monitor the state of it’s indexing in Google and other search engines, you’ve undoubtedly run into the problem of duplicate content. The most basic manifestation of this problem could be the indexing of both the www and non-www versions of your website. If your website is dynamic, there is a good chance that the complex URLs produced lead to duplicate content being indexed by the search engines.
The canonical link tag, which is supported by Google, Yahoo and Microsoft is placed in the head of your web page document. It tells the search engine the preferred URL that should be used to index the content. Here is an example:
<link rel=”canonical” href=”http://www.yoursite.com/product.php?item=blue-widget” />
Obviously, if your web pages are being produced dynamically (I don’t really see a use for this tag for simple sites where they are not) you are going to need to add some kind of functionality that produces the new tag. There are already plugins available for WordPress, Drupal and Magento packages.
THe canonical link tag certainly seems like a good idea, but we’ll have to see how smoothly the implementation on websites plays out, and if it lives up to it’s potential in improving search engine performance and ease of maintenance for those sites that use it.