The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:string-replace match="/" replace="'text'">
<p:with-input>
<p:inline document-properties="map{'serialization':map{}, 'bonus-prop':'prop', 'base-uri': 'http://base'}">
<doc/>
</p:inline>
</p:with-input>
</p:string-replace>
<p:cast-content-type content-type="application/xml">
<p:with-input select="p:document-properties(.)"/>
</p:cast-content-type>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:ns prefix="j" uri="http://www.w3.org/2005/xpath-functions"/>
<s:pattern>
<s:rule context="/">
<s:assert test="j:map">Root element is not 'j:map'.</s:assert>
<s:assert test="j:map/j:string[@key='base-uri']/text()='http://base'">Base-uri properties is not 'http://base'.</s:assert>
<s:assert test="j:map/j:string[@key='bonus-prop']/text()='prop'">Property 'bonus-prop' is not preserved.</s:assert>
<s:assert test="j:map/j:string[@key='content-type']/text()='text/plain'">Content type is not 'text/plain'.</s:assert>
<s:assert test="not(j:map/*[@key='serialization'])">There should be no element 'serialization'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
