HTML - Canonical URL

What is a Canonical URL

A canonical url is a URL that has a canonical value identifier for a web page meaning that the value should be unique on the internet scope.

This is the URL that people will see in:

The Facebook graph uses this value as identifier for a web page

The same page may have different URL such as:

The link rel canonical defines which one is the good identifier and don't let robot such as google search defines it by chance.

The same page may be published in a category

The same page may be syndicated (published on another site)

A page may be translated.

A Mobile page and desktop page may not be served from the same domain and link canonical permits to identify them as the same resource.

Moving a page to another URL without losing integrated components such as embedded discussion components.

A Rel canonical is a tag in the header in a HMTL page that tells search engines:

which piece of content is the original and which one is a duplicate. It has been created in order to avoid duplicate content
which operates exclusively on a single root domain (it will carry over across subfolders and subdomains, not cross domain)

A link rel canonical is a link html element with a rel attribute.

<link rel="canonical" href="https://datacadamia.com/cat/post1" />

Once created the value https://datacadamia.com/cat/post1 should never change even if the page get a new URL

Make sure that you use:

for the canonical href value.

The pages do not need to be absolutely identical. They just need to serve the same content.

The og:url open graph property served the same goal. Facebook uses it

<meta property="og:url" content="https://example.com/path" />