Starting from JDK 9, LSSerializer skips the default namespace. The following test fails in JDK 9 and later:

    public void testDefaultNS() throws Exception {
        String xml = "<a "
                + "xmlns=\"http://openjdk_java_net/xml/defaultNS\" "
                + "xmlns:p1=\"http://openjdk_java_net/xml/serializer/\">"
                + "<b>in default namespace</b></a>";
        DocumentBuilder db = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        LSSerializer ls = getSerializer(db);
        String out = ls.writeToString(getDoc(db, xml));
        Assert.assertTrue(out.contains("defaultNS"));
    }
    
    private LSSerializer getSerializer(DocumentBuilder db) throws Exception {
        DOMImplementationLS di = (DOMImplementationLS) db.getDOMImplementation();
        return di.createLSSerializer();
    }
    
    private Document getDoc(DocumentBuilder db, String xml) throws Exception {
        InputSource is = new InputSource(new StringReader(xml));
        return db.parse(is);
    }