/**
  * @return SgmlOpenTag
  **/
 private function createOpenTag()
 {
     if (!self::isValidId($this->tagId)) {
         $this->error("tag id '{$this->tagId}' is invalid");
     } elseif ($this->lowercaseTags) {
         $this->tagId = strtolower($this->tagId);
     }
     return $this->setupTag(SgmlOpenTag::create());
 }