I often write XML files using the "xml-stylesheet" 
processing instruction where the stylesheet is located at 
my local drive relative to the location of the XML file.  
I'm also using DTD which is relative to it's location.

When I open the XML file using Microsoft Internet Explorer 
as external browser, the MSXML rendering engine in IE 
doesn't find the relative resources (XSL and DDT) when the 
file's URL points to internal netbeans web server.

I thus suggest that an XML file in a LocalFilesystem will 
be displayed with the actual file URL insted of the local 
web servers URL.

The following changes in the view() method of 
org.netbeans.modules.xml.core.XMLDataObject.ViewSupport
will do the trick:

        public void view () {
            try {
                org.openide.filesystems.FileSystem 
fileSystem = primary.getFile().getFileSystem();
                if (fileSystem instanceof 
org.openide.filesystems.LocalFileSystem) {
                    File file = 
((org.openide.filesystems.LocalFileSystem)
fileSystem).getRootDirectory();
                    StringTokenizer strTok = new 
StringTokenizer(primary.getFile().toString(), "/");
                    while(strTok.hasMoreTokens()) {
                        file = new File(file, 
strTok.nextToken());
                    }
                    TopManager.getDefault ().showUrl 
(file.toURL());
                } else {
                   TopManager.getDefault ().showUrl 
(primary.getFile().getURL());
                }
            } catch (Exception e) {
            }
        }