code: function serializeDoctype (node) {
    var r = '<!DOCTYPE ' + node.name;
  
    if (node.publicId) {
      r += ' PUBLIC "' + node.publicId + '"';
    }
  
    if (!node.publicId && node.systemId) {
      r += ' SYSTEM';
    }
  
    if (node.systemId) {
      r += ' "' + node.systemId + '"';
    }
  
    r += '>';
    return r;
  }
docs: Serialize a DOCTYPE node.
  See: http://stackoverflow.com/a/10162353