It requires the following features: p:http-request.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:http-request href="http://localhost:8246/service/fixed-alternative">
<p:with-input>
<p:empty/>
</p:with-input>
</p:http-request>
<p:for-each>
<p:choose>
<p:when test="p:document-property(., 'content-type')='text/plain'">
<p:wrap-sequence wrapper="text-wrapper"/>
</p:when>
<p:otherwise>
<p:cast-content-type content-type="application/xml"/>
</p:otherwise>
</p:choose>
</p:for-each>
<p:wrap-sequence wrapper="result"/>
</p:declare-step>
Schematron validation
<s:schema xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:ns prefix="html" uri="http://www.w3.org/1999/xhtml"/>
<s:ns prefix="c" uri="http://www.w3.org/ns/xproc-step"/>
<s:pattern>
<s:rule context="/">
<s:assert test="result">The root element is not 'result'.</s:assert>
<s:assert test="count(result/html:html)=2">There is two children 'html:html'.</s:assert>
<s:assert test="count(result/html:html/html:head/html:title[. ='Hello world!'])=2">There are not two documents with title 'Hello world!'</s:assert>
<s:assert test="count(result/html:html/html:body/html:h1/html:img[@src='helloworld.png'])=2">There a is not two document with 'helloworld.png' in body/h1/img.</s:assert>
<s:assert test="result/c:data">The root element is not 'c:data'.</s:assert>
<s:assert test="result/c:data/@content-type='image/png'">Content-Type is not 'image/png'.</s:assert>
<s:assert test="result/c:data/@encoding='base64'">Encoding is not 'base64'.</s:assert>
<s:assert test="starts-with(result/c:data/text(),'iVBORw0KGgoAAAANSUhEUgAAAPwAAABJCAYAAAAZrEuhAAAAAXNSR0IArs4c6QAAAAZiS0dEAP8A') and ends-with(result/c:data/text(), '/f8AMGN9ADsjEr4AAAAASUVORK5CYII=')">Content is not right.</s:assert>
<s:assert test="starts-with(result/text-wrapper/text(),'Hello world!') and contains(result/text-wrapper/text(),'This is a hello world document.')">There is no text document with the right text.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
