Canonical XML is a normal form of XML, intended to allow relatively simple comparison of pairs of XML documents for equivalence; for this purpose, the Canonical XML transformation removes non-meaningful differences between the documents. Any XML document can be converted to Canonical XML.

For example, XML permits whitespace to occur at various points within start-tags, and attributes to be specified in any order. Such differences are seldom if ever used to convey meaning, and so these forms are generally considered equivalent:

In converting an arbitrary XML document to Canonical XML, attributes are encoded in a normative order (alphabetical by name), and with normative spacing and quoting. Thus, the second form above would be converted to the first.

Canonical XML specifies a number of other details, some of which are:

Converting a document to Canonical XML is idempotent. That is, the first conversion usually will result in a different string of characters than the original, but repeated conversions will make no further changes.