I found that the Last Call working draft of xml:id was out. But I think xml:id will be incompatible with Canonical XML (xml-c14n). Below is an excerpt from 2.4 Document Subsets in xml-c14n W3C REC:
The processing of an element node E MUST be modified slightly when an XPath node-set is given as input and the element's parent is omitted from the node-set. The method for processing the attribute axis of an element E in the node-set is enhanced. All element nodes along E's ancestor axis are examined for nearest occurrences of attributes in the xml namespace, such as xml:lang and xml:space (whether or not they are in the node-set). From this list of attributes, remove any that are in E's attribute axis (whether or not they are in the node-set). Then, lexicographically merge this attribute list with the nodes of E's attribute axis that are in the node-set. The result of visiting the attribute axis is computed by processing the attribute nodes in this merged attribute list.
Well, I don't think xml:id is wrong here. It is xml-c14n that is based on non-committed premises that all xml:* attributes must be inherited (yes, xml:lang, xml:space and xml:base were). Anyways, don't worry about that incompatibility. Canonical XML is already incompatible with XML Infoset with related to namespace information items.
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