Created attachment 34894 [details]
Reproducer war

When you try to use references to local xslt files inside same deployment.
You can get error when creating XML Transformer as transformer factory doesn't have URI resolver set to be able to recognize jstl protocol.

Reproducer "test.war" is attached.
Part of code that makes it fail:

test.jsp:
------------
<c:import var="xslStylesheet" url="/xsl/style.xsl" charEncoding="UTF-8" />
<c:import url="http://localhost:8080/test/test.xml" var="doc"/>
<x:transform xml="${doc}" xslt="${xslStylesheet}" xsltSystemId="/xsl/">
</x:transform> 

style.xsl
---------------
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                version="1.0">

    <xsl:import href="parent.xsl"/>
...

results in
javax.xml.transform.TransformerException: Had IO Exception with stylesheet file: parent.xsl
	   at org.apache.xalan.processor.TransformerFactoryImpl.newTemplates(TransformerFactoryImpl.java:942)
	   at org.apache.xalan.processor.TransformerFactoryImpl.newTransformer(TransformerFactoryImpl.java:780)
	   at __redirected.__TransformerFactory.newTransformer(__TransformerFactory.java:133)
   at org.apache.taglibs.standard.util.XmlUtil.newTransformer(XmlUtil.java:187)
Caused by: java.net.MalformedURLException: unknown protocol: jstl
   at java.net.URL.<init>(URL.java:600)
   at java.net.URL.<init>(URL.java:490)
   at java.net.URL.<init>(URL.java:439)
   at org.apache.xerces.impl.XMLEntityManager.setupCurrentEntity(XMLEntityManager.java:964)
   at org.apache.xerces.impl.XMLVersionDetector.determineDocVersion(XMLVersionDetector.java:144)
   at org.apache.xerces.parsers.XML11Configuration.parse(XML11Configuration.java:832)
   at org.apache.xerces.parsers.XML11Configuration.parse(XML11Configuration.java:798)
   at org.apache.xerces.parsers.XMLParser.parse(XMLParser.java:108)
   at org.apache.xerces.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1198)
   at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:564)
   at org.apache.xalan.processor.ProcessorInclude.parse(ProcessorInclude.java:312)

This is also tracked in Red Hat BZ https://bugzilla.redhat.com/show_bug.cgi?id=1320747
as well as in WildFly Jira: https://issues.jboss.org/browse/WFLY-8498