code: public static void switchSerializerIfHTML(
            TransformerImpl transformer, String ns, String localName)
              throws TransformerException
    {
  
      if (null == transformer)
        return;
  
      if (((null == ns) || (ns.length() == 0))
              && localName.equalsIgnoreCase("html"))
      {
        // System.out.println("transformer.getOutputPropertyNoDefault(OutputKeys.METHOD): "+
        //              transformer.getOutputPropertyNoDefault(OutputKeys.METHOD));     
        // Access at level of hashtable to see if the method has been set.
        if (null != transformer.getOutputPropertyNoDefault(OutputKeys.METHOD))
          return;
  
        // Getting the output properties this way won't cause a clone of 
        // the properties.
        Properties prevProperties = transformer.getOutputFormat().getProperties();
        
        // We have to make sure we get an output properties with the proper 
        // defaults for the HTML method.  The easiest way to do this is to 
        // have the OutputProperties class do it.
        OutputProperties htmlOutputProperties = new OutputProperties(Method.HTML);
  
        htmlOutputProperties.copyFrom(prevProperties, true);
        Properties htmlProperties = htmlOutputProperties.getProperties();
  
        try
        {
  //        Serializer oldSerializer = transformer.getSerializer();
          Serializer oldSerializer = null;
  
          if (null != oldSerializer)
          {
            Serializer serializer =
              SerializerFactory.getSerializer(htmlProperties);
  
            Writer writer = oldSerializer.getWriter();
  
            if (null != writer)
              serializer.setWriter(writer);
            else
            {
              OutputStream os = oldSerializer.getOutputStream();
  
              if (null != os)
                serializer.setOutputStream(os);
            }
  
  //          transformer.setSerializer(serializer);
  
            ContentHandler ch = serializer.asContentHandler();
  
            transformer.setContentHandler(ch);
          }
        }
        catch (java.io.IOException e)
        {
          throw new TransformerException(e);
        }
      }
    }
docs: Switch to HTML serializer if element is HTML
  
  
  @param transformer Non-null transformer instance
  @param ns Namespace URI of the element
  @param localName Local part of name of element
  
  @throws TransformerException