Add the standard-defined "xml" namespace if it is not specified
Add the standard-defined "xml" namespace if it is not specified

The XML standard stipulates:
The prefix xml is by definition bound to the namespace name http://www.w3.org/XML/1998/namespace. It MAY, but need not, be declared, and MUST NOT be bound to any other namespace name. Other prefixes MUST NOT be bound to this namespace name, and it MUST NOT be declared as the default namespace.

If the document does not explicitly include this namespace, it is added. This prevents errors due to the use of the namespace in some imported SVG files. In debug builds those errors emit warning messages, and in Windows debug builds those errors cause an abort() to be called.

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