Mobile SEO: The Canonical Tag

One of the biggest problems with the current mobile search engine algorithm is that it has a difficulty understanding which page to rank when both a mobile and a traditional version of the page is available. If you leave it up to the search engines, even the mobile search engines tend to pick the traditional version of a page. This is because algorithmically, the traditional pages have more history and links than their newer mobile counterparts. To take advantage of the traditional page rankings, but still show the mobile pages to visitors, you can set up user-agent detection and redirection on your server.

If user-agent detection and redirection is set up correctly, it will ensure that when the traditional pages out-rank the mobile pages in the search engines, users who click on those traditional results will be delivered seamlessly to the mobile page instead. If you have done that and have also done a good job on mobile file naming you can take this strategy to the next level with a strategic implementation of the canonical tag.

Why We Use Canonical Tags

Canonical tags were created for a similar but different set of circumstances. They are a way to tell the search engines which url should rank when you knowingly have multiple versions of a page. Essentially, you are saying β€œall these pages are basically the same, so this [specific url] is the copy that we would like you to rank.”

Having a canonical strategy on your mobile site can actually help with traditional SEO as well. The basic idea is that – if you have user-agent detection and re-direction set up properly, there is no real need to push SEO value to the mobile pages on their own – at least for the majority of searches. While the mobile-specific pages will have a better chance of ranking well on feature phones, a vast majority of the mobile search traffic is from smart phones, which seem to favor the traditional pages.

For this strategy, you actually add a canonical tag that references the traditional version of the same page into the header of the mobile page. This tells the search engine that the two pages are essentially the same, but that your preference is that they rank the traditional one. For this strategy, it is important to go page-to-page, rather than page-to-home-page. That means this strategy is best used when you have a one-to-one ratio of mobile pages to traditional pages, or when it is quite easy to match up mobile and traditional page urls. Without that, it can be a bit of a messy process.

With this canonical strategy on your mobile site, you want the mobile pages to be optimal and indexiable, but you rely most heavily on the traditional pages for rankings. The canonical tag is just a suggestion, rather than a mandate, so if the search engines see value in keeping your mobile pages indexed separately, they will. Otherwise, the search engines will consolidate the SEO value of the mobile and traditional pages, which will be powerful in mobile AND traditional search results.

Here is precisely how Google instructs us to implement the canonical tag, in their words:
(source: http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html)

Now, you can simply add this <link> tag to specify your preferred version:

<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />

inside the <head> section of the duplicate content URLs:

http://www.example.com/product.php?item=swedish-fish&category=gummy-candy
http://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

and Google will understand that the duplicates all refer to the canonical URL: http://www.example.com/product.php?item=swedish-fish. Additional URL properties, like PageRank and related signals, are transferred as well.

Canonical tags are taken as a medium-strength suggestion. They are not a directive, but they can help pass any SEO value, links, relevance or history that the mobile page accumulates to its traditional counterpart, which is at a natural advantage anyway. Pushing the mobile SEO value to the traditional page should push SEO value for both smart phone mobile search results and traditional search results, because it will help consolidate the value of the two pages.

This strategy is only ideal if you are in a situation where mobile pages are not ranking well or driving much traffic on their own, and you are targeting primarily a smart phone audience. If the rankings of your mobile content are already good, you are targeting a large portion of traffic from feature phones or if you are not confident in your user-agent detection, then this strategy should be approached very cautiously, or avoided. Since you are relying on the rankings of the traditional pages, the risk is that the canonical tag to the traditional page could hurt existing rankings of the mobile pages in searches on less sophisticated phones that would be able to display the traditional site at all, like in feature phones or WAP phones.