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Canonical link element

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Purpose and Background
– Search engines face the challenge of determining the original source for documents available on multiple URLs.
– Content duplication can occur due to GET-parameters, multiple URLs from CMS, accessibility on different hosts/protocols, and print versions of websites.
– Duplicate content issues arise when the same content is accessible from multiple URLs.
Google, Yahoo, and Microsoft introduced support for the canonical link element in 2009 to address this problem.
– The canonical link element helps webmasters indicate the original page that should be credited.

How search engines handle rel=canonical
– Search engines use canonical link definitions as an output filter for search results.
– The canonical link URL definitions help determine the original source of content when multiple URLs have the same content.
Google considers the canonical link element as a hint that its ranking algorithm strongly honors.
Matt Cutts, former head of Google’s webspam team, stated that Google prefers the use of 301 redirects over canonical link elements.
– The choice of which resource to display in search results depends on the search query.

– The canonical link element can be used in the semantic HTML head section or sent with the HTTP header of a document.
– For non-HTML documents, the HTTP header provides an alternate way to set a canonical URL.
– According to the HTML 5 standard, the link rel=canonical href= HTML element must be within the head section of the document.
– Some websites, like Stack Overflow, use self-hyperlinks to link to a clean URL of themselves.
– Self-hyperlinks offer usability benefits such as easy copying of the hyperlink target URL or title.

HTML example: The rel=canonical is used inside the head tag to indicate the preferred version of a webpage.
– HTTP example: The HTTP header includes a canonical link in the response to specify the preferred URL.
URL normalization is related to canonical link elements.
– The canonical link element is a way to address duplicate content issues in search engine optimization.
– The use of canonical tags helps prevent duplicate content clutter in search engine results.

Additional Resources
URL normalization is a related topic to explore further.
– The Search Console Help provides guidance on how to consolidate duplicate URLs.
– The HTML link tag is a useful resource for understanding HTML elements.
– Meta Stack Exchange discusses the question title linking to itself on the answer page.
Search Engine Journal recommends three Firefox addons for easier copying of links and anchor texts.

A canonical link element is an HTML element that helps webmasters prevent duplicate content issues in search engine optimization by specifying the "canonical" or "preferred" version of a web page. It is described in RFC 6596, which went live in April 2012.

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