rel="canonical" attribute value on a <link> tag specifies the preferred URL for the current page. This attribute value prevents duplicate URL issues.
<head> <link rel="canonical" href="www.mycompany.com/products"> </head>
The rel attribute defines the relationship between the current page and the linked page or resource.
rel="canonical" value tells search engines that the href value is the 'official' URL of the page.
The biggest benefit is for SEO purposes as this prevents multiple URLs for the same content or page.
Potential duplicates for url www.mycompany.com/products include:
All are valid urls referencing the same content, but automatic readers see these urls as different.
Building consistent URLs and avoiding duplicates increases the quality of SEO data that can be collected.
<link rel="canonical" href="url" />
Only one element accepts the canonical value on the rel attribute.
|<link>||Specifies a resource to the current page -- see example above|
Here is when rel support started for each browser: