An XQuery 1.0 implementation may choose to support either XML 1.0 and Namespaces 1.0, or XML 1.1 and Namespaces 1.1. This is an implementation-defined choice that should be clearly documented. XML 1.1 allows a much wider set of characters in XML names, and adds two line-end characters to the set of characters that are considered to be whitespace. The main changes in Namespaces 1.1 are the ability to undeclare prefixes, and support for Internationalized Resources Identifiers (IRIs) rather than just URIs.

In addition, an XQuery implementation can choose what version of Unicode to support for the functions that rely on Unicode definitions, such as normalization and case mapping.