It's been several months since the link canonical tag was announced, and it's being used fairly liberally out in the wild. It's patently clear to us that this tag is quite powerful and effective, and the consequences of its misuse very serious. It's being misused a lot (not a surprise). We're seeing a ton of sites with poor rel=canonical implementation. The end result: it's causing havoc.
Yes, rel=canonical is breaking websites. I'll share a few anecdotes in this post that show just how bad it can be.
Why Link Canonical is Dangerous
Part of the problem with rel=canonical is that it's extremely easy to implement. Just throw a meta tag into the head of a page and you're good. That very ease belies the power of the tag. Google announced to us at SMX East last October, that 2 out of 3 times the link canonical target influences the organic decsion. That's right -- 2/3 of the time your rel=canonical target is affecting the crawl and indexation of the page, and in turn, the ranking of the page.
You can see how easy it is to mess this up. Now, when clients tell me they've "already got plans for the link canonical tag" I get all weird and anxious on the phone... it scares the heck out of me. At least with hard redirects you can visually see the change occur. With the link canonical in place, you don't see anything, unless you look at the source. It's like nothing happened at all.
You're Doing it Wrong
At SES Chicago this month on the Duplicate Content panel, I shared the story of how a client of ours with about 100,000 SKUs on one of their sites had somehow put a link canonical target of the home page on every single page. Every product page, every category, every section, literally every page pointed back with rel=canonical to the root domain. We didn't realize this had happened until 2 or 3 months after the fact, because we weren't working on that particular site at the time. You can imagine what the traffic profile looked like.
Bruce Clay told me recently, that "we're recommending that the link canonical be implemented only with professional help." Amen to that.
Other failures we've seen with implementations of link canonical tags:
- The link canonical points to itself. This is fine when there are no other options during implementation, but not advised for sitewide usage because it can introduce unexpected behaviour. Be careful with this one.
- A link canonical chain is created, with canonical targets pointing to multiple URLs, and back and forth, becoming a web of confusion. For example: the http://www.mydomain.com link canonical points to http://www.mydomain.com/index.html and that one points back to the canonical version. Choose a canonical version when duplication exists. Stick with it - consistency is key.
- Link canonicals on deep pages (such as product pages) point to the category or parent URL.
Now that Google is supporting cross-domain usage of rel=canonical, which is fabulous news for advanced SEOs, I imagine it's going to be even worse out there. Please people, be careful using this tag, and get professional help. Incorrect use of rel=canonical has serious implications.