2.2.1 [XML] Section 2.3, Common Syntactic Constructs
The specification states:
The Namespaces in XML Recommendation [XML Names] assigns a meaning to names containing colon characters. Therefore, authors should not use the colon in XML names except for namespace purposes, but XML processors must accept the colon as a name character.
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Colons are accepted in names only when they are used in accordance with [NamespacesXML1.1].