Names beginning with the string "xml", or with any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.

from here https://www.w3.org/TR/xml/#sec-common-syn

so I think the following

<rest:sequence xmlns:rest="https://sirix.io/rest"> <rest:item> <xml rest:id="1"> foo <bar rest:id="3"> <test rest:id="4"> yikes <bar rest:id="6"/> </test> </bar> </xml> </rest:item> </rest:sequence>

is not well-formed, or has there been some change I've missed?