The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:delete match="element/@xml:base" name="delete">
<p:with-input>
<doc>
<element xml:base="http://dummy-base-uri"/>
</doc>
</p:with-input>
</p:delete>
<p:identity name="base-uri">
<p:with-input>
<base-uri>{base-uri(doc/element)}</base-uri>
</p:with-input>
</p:identity>
<p:insert match="/doc/element" position="first-child">
<p:with-input port="source" pipe="@delete"/>
<p:with-input port="insertion" pipe="@base-uri"/>
</p:insert>
</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="doc">The document root is not 'doc'.</s:assert>
<s:assert test="doc/element">There is no element 'element' as child of doc.</s:assert>
<s:assert test="count(doc/element/@xml:base)=0">There should be not attribute 'xml:base' on 'element'.</s:assert>
<s:assert test="doc/element/base-uri/text()='http://dummy-base-uri'">Text of element 'element' is not 'http:/dummy-base-uri'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
