A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical" or "preferred" version of a web page as part of search engine optimization. It is described in RFC 6596, which went live in April 2012.
A major problem for search engines is to determine the original source for documents that are available on multiple URLs. Content duplication can happen in a lot of ways. The most common reasons for duplication are:
Duplicate content issues occur when the same content is accessible from multiple URLs. For example, http://www.example.com/page.html would be considered by search engines to be an entirely different page from http://www.example.com/page.html?parameter=1, even though both URLs return the same content. Another example is essentially the same (tabular) content, but sorted differently.
Canonical tags can also be useful to solve www and non-www duplicate content--where two URLs, identical except that one begins with "www" and the other does not point to the same page. This particular problem can be solved by proper use of the
In February 2009, Google, Yahoo and Microsoft announced support for the
canonical link element, which can be inserted into the <head> section of a web page, to allow webmasters to prevent these issues. The canonical link element helps webmasters make clear to the search engines which page should be credited as the original.
Search engines try to utilise canonical link definitions as an output filter for their search results. If there is more than one URL with the same content (duplicate content) in the result set, the canonical link URL definitions will likely be incorporated to determine the original source of the content.
While the canonical link element has its benefits, Matt Cutts, the head of Google's webspam team, has claimed that the search engine prefers the use of 301 redirects. Cutts claims the preference for redirects is because Google's spiders can choose to ignore a canonical link element if they deem it more beneficial to do so.
The canonical link element can be either used in the HTML <head>, or sent with the HTTP header of a document. For non HTML documents, the HTTP header is an alternate way to set a canonical URL.
By the HTML 5 standard, the
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<html> <head> <link rel="canonical" href="http://example.com/" /> </head> <body> ...<ref>sdfdf</ref>
<link rel="canonical" href="http://example.com/index.php" />
HTTP/1.1 200 OK Content-Type: application/pdf Link: <http://example.com/page.html>; rel="canonical" Content-Length: 4223 ...
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