gecko-dev/devtools/server/tests/browser/test-window.xhtml

<?xml version="1.0" encoding="UTF-8"?>
<xul:window xmlns="http://www.w3.org/1999/xhtml"
            xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
            title="Test page">
</xul:window>