About
A canonical form is a unique representation such that every object can be uniquely identified in the global scope
A canonical form specifies a unique representation for every object (entity), while a normal form simply specifies its form, without the requirement of uniqueness.
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Usage
In computer science, the equality of two objects can easily be tested by testing the equality of their canonical forms.
HTML
In html, the canonical form of an document entity is found in the link rel canonical