2.4.1 Document Order

"An ordering called document order is defined among all the nodes accessible during processing of a given expression, which may consist of one or more trees (documents or fragments). Document order is defined in [XQuery and XPath Data Model (XDM) 3.1], and its definition is repeated here for convenience. Document order is a total ordering, although the relative order of some nodes is implementation-dependent. [Definition: Informally, document order is the order in which nodes appear in the XML serialization of a document.] "

Correct link for XDM: http://www.w3.org/TR/xpath-datamodel-31/#document-order 2.4 Document Order

BTW, the definition in XDM reads: 

"[Definition: A document order is defined among all the nodes accessible during a given query or transformation. Document order is a total ordering, although the relative order of some nodes is implementation-dependent. Informally, document order is the order in which nodes appear in the XML serialization of a document.]" 

Compare: 

 [Definition: Informally, document order is the order in which nodes appear in the XML serialization of a document.] "

I would simply insert the reference and/or quote the relevant definition rather than attempting to recast it.

[Will this make parsers faint and strong users curse? No but it is symptomatic.]