It requires the following features: xslt-3.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:xslt version="3.0" name="xslt">
<p:with-input port="source">
<doc/>
</p:with-input>
<p:with-input port="stylesheet">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="3.0">
<xsl:output build-tree="yes"/>
<xsl:template match="/">
<xsl:sequence select="1, 'a', true()"/>
</xsl:template>
</xsl:stylesheet>
</p:with-input>
</p:xslt>
<p:wrap-sequence wrapper="result"/>
<p:add-attribute match="result" attribute-name="content-type">
<p:with-option name="attribute-value" select="p:document-property(.,'content-type')">
<p:pipe step="xslt"/>
</p:with-option>
</p:add-attribute>
</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 document root is not 'result'.</s:assert>
<s:assert test="result/text()='1 a true'">Element 'result' does not have a text child '1 a true'.</s:assert>
<s:assert test="result/@content-type='text/plain'">Element 'result' does not have an attribute 'content-type' with 'text/plain'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
