The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:input port="source">
<p:inline document-properties="map{'base-uri' : 'http:/dummy-base-uri'}">
<?pi target?>
</p:inline>
</p:input>
<p:viewport match="processing-instruction()" name="viewport">
<p:identity>
<p:with-input>
<result>{p:document-property(., 'base-uri')}</result>
</p:with-input>
</p:identity>
</p:viewport>
</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:pattern>
<s:rule context="/">
<s:assert test="result">The root element is not 'result'.</s:assert>
<s:assert test="result/text()='http:/dummy-base-uri'">The base uri is not correctly set.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
