rel-canonical

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(See Also: add RFC)
(The opposite of canonical)
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=== Cross-domain links ===
=== Cross-domain links ===
Although the primary use case is links within the same domain, rel=canonical can be used [https://webmasters.googleblog.com/2009/12/handling-legitimate-cross-domain.html across domains] too. A common use case recently is from [https://www.ampproject.org/docs/guides/deploy/discovery cached AMP pages] to their original.
Although the primary use case is links within the same domain, rel=canonical can be used [https://webmasters.googleblog.com/2009/12/handling-legitimate-cross-domain.html across domains] too. A common use case recently is from [https://www.ampproject.org/docs/guides/deploy/discovery cached AMP pages] to their original.
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== The opposite of canonical ==
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There are several ways of indicating non-canonical versions of a page, depending on what is intended. A non-comprehensive list
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=== rel-syndication and u-syndication ===
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[[rel-syndication]] and [http://microformats.org/wiki/h-entry#Core_Properties u-syndication] link to copies that the author has syndicated or shared onto other sites. The syndicated copies {{should}} have a rel-canonical link back to the original if the other site allows it.
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=== rel-alternate===
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[[rel-alternate]] is used to link to versions of the page in other forms, whether a different format with <code>type</code>, language with <code>hreflang</code> or presentation medium with <code>media</code>.
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=== rel-amphtml ===
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[[rel-amphtml]] is used to link to the Accelerated Mobile Pages version, a html subset limiting javascript, replacing built-in elements with custom elements and designed for caching. It is [https://www.ampproject.org/docs/guides/deploy/discovery recommended] to link back fromt he AMP version to the original with rel-canonical
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=== rel-shortlink ===
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[[rel-shortlink]] is used to link to a url shortened version designed for sharing and (often) tracking.
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=== rel-me ===
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[[rel-me]] is explicitly saying that another domain or page  represents the same person or organisation as the current page. As such it is a different level of abstraction from rel-canonical, as it refers to the representation rather than the content.
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=== rev-canonical ===
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A deprecated way of linking to a short link, briefly [https://revcanonical.wordpress.com/ popular], [https://www.mnot.net/blog/2009/04/14/rev_canonical_bad disliked] and not recommended.
== Issues ==
== Issues ==

Revision as of 10:58, 28 March 2017

Kevin Marks (Editor)


rel=canonical is a link relation to indicate the canonical URL of the current page, to avoid duplicate content.

Per CC0, to the extent possible under law, the editors have waived all copyright and related or neighboring rights to this work. In addition, as of 2017-10-18, the editors have made this specification available under the Open Web Foundation Agreement Version 1.0.

Contents

Abstract

By adding rel="canonical" to a hyperlink, a page indicates that the destination of that hyperlink SHOULD be considered the preferred or definitive version of the current page. This helps search engines avoid duplicate content, and is useful for deciding how to link to a page when citing it.

Discussion

There are many reasons for a site to serve the same content at multiple URLs, but duplicate pages are undesirable in search results. The historic recommendation was to use 301 redirects to the canonical page, but that can be jarring for users. There are many use cases for a URL to have additional information added as query parameters or fragments to indicate the navigation history for referral purposes, but would prefer a single main referent from searches. This was codified in February 2009 as rel=canonical, and adopted by many search engines.

Search engines prefer rel=canonical on a <link> element, and will ignore it on an <a> element.

Cross-domain links

Although the primary use case is links within the same domain, rel=canonical can be used across domains too. A common use case recently is from cached AMP pages to their original.

The opposite of canonical

There are several ways of indicating non-canonical versions of a page, depending on what is intended. A non-comprehensive list

rel-syndication and u-syndication

rel-syndication and u-syndication link to copies that the author has syndicated or shared onto other sites. The syndicated copies SHOULD have a rel-canonical link back to the original if the other site allows it.

rel-alternate

rel-alternate is used to link to versions of the page in other forms, whether a different format with type, language with hreflang or presentation medium with media.

rel-amphtml

rel-amphtml is used to link to the Accelerated Mobile Pages version, a html subset limiting javascript, replacing built-in elements with custom elements and designed for caching. It is recommended to link back fromt he AMP version to the original with rel-canonical

rel-shortlink

rel-shortlink is used to link to a url shortened version designed for sharing and (often) tracking.

rel-me

rel-me is explicitly saying that another domain or page represents the same person or organisation as the current page. As such it is a different level of abstraction from rel-canonical, as it refers to the representation rather than the content.

rev-canonical

A deprecated way of linking to a short link, briefly popular, disliked and not recommended.

Issues

See Also

rel-canonical was last modified: Wednesday, December 31st, 1969

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