Implementing canonical tags helps to avoid duplicate content issues and indicate what pages you'd like to see in search results. However, it brings results just in case you implement canonical tags correctly. There are a lot of misconceptions about canonicalization which lead to a completely opposite of what you expect.
In this guide, we'll discuss the most common canonicalization mistakes with rel=canonical so you can avoid them while working on your SEO strategy.
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If you think things like typos are your biggest problem when you add canonical tags to your website, I have bad news for you. Sometimes mistakes with rel=canonical are hidden in such plain sight it is hard to spot them.
Nevertheless, here are the most common canonicalization errors you should know about and avoid.
1. Wrong relative URLs
Like many other HTML tags, <link> tags also accept absolute and relative URLs. However, when applying canonical URLs many make the mistake of defining absolute URLs incorrectly.
Relative URLs include only the "relative "path, while absolute — the full path to the resources page which is differentiated by the http:// schema.
In simple words, if you include the full website URL without the http:// it will cause an indexing error and rel=canonical might even be ignored. Use either relative or absolute URL, but make sure you do it the right way.
2. Multiple rel=canonical tags
One of the most common canonicalization mistakes is using multiple canonical tags for the same page. They could be added by a theme, SEO extensions, your CMS, etc. Regardless of the reason, having multiple declarations of rel=canonical sends wrong signals to the search crawlers. As a result, all of the canonicals will be ignored altogether.
3. Canonical tags in <body>
If you don't want your rel=canonical to be disregarded, make sure to add it to the <head> of page HTML, NOT <BODY>. It is also good to include the canonical URL early in the <head> section to avoid it being thrown into <body> during parsing.
4. Canonicalizing all pages in a series to the root page
The rel=canonical tags are used to define the main version of the page to avoid duplicate content. However, content in a series of pages is not duplicated. Correspondingly you can't add rel=canonical from all pages of a series to the first page unless you want them to be ignored by search crawlers.
This canonicalization error most often appears on category pages. So if you can't add links from each page of a series to the "view all" page is it recommended to use How to Add link rel="next" and rel="prev" in Magento 2? pagination.
5. Invalid canonicalization instructions
Make sure all URLs you canonicalize are not blocked in robots.txt or used together with the noidex tag. If the link is blocked in robots.txt, no search crawlers will be able to crawl and index it.
Moreover, if you use noindex tag and canonical tag it will send contradictory instructions to the search crawlers. And though search engines will prioritize canonical tag over noindex, it is still not recommended to use both.
The same goes for the 4XX HTTP status code. Make sure canonicalized links don't return 4XX HTTP status code so search engines can see the rel=canonical and transfer the "link equity" to it correspondingly.
6. Incorrect hreflang tags implementation
If you manage a multi-language store you've probably heard about hreflang tags and how they work. However, you should also know about the canonicalization error that appears if you use hreflangs with canonical tags.
For those who don't know, hreflang tags tell search engines about the language alternatives of the same page. So hreflang tags tell search bots to follow and index all of those pages while canonical tags tell them to index only the main version of the page.
If you canonicalize only one page in a series of alternate pages while hreflang tags specify there are different alternatives of the page, it will only confuse search engines. So, each alternative language page should contain a self-referencing canonical.
Canonical tags play an important part in your SEO strategy since they define what pages you'd like to be pulled out in search. While these rel=canonical mistakes don't look too complicated they can cause you a lot of trouble, especially if you have a lot of pages to edit the canonical for.
That's why we recommend resorting to the solutions that manage canonical tags automatically.
If your store is based on Magento, check out Magento 2 SEO Extension which covers canonical tags on all of your website pages and plenty of other vital SEO-related features.