The original XML document definition from 1999 states
The attribute's value, a URI reference, is the namespace name identifying the namespace. The namespace name, to serve its intended purpose, should have the characteristics of uniqueness and persistence. It is not a goal that it be directly usable for retrieval of a schema (if any exists).
It should suffice for the namespace (not the prefix) to be an unique string, but in this case you will usually get a warning like
namespace warning : xmlns: URI ... is not absolute
By convention, it should be a
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