code: protected final function formatTagPrefix(Element $element)
      {
          $tag  = "<";
          $tag .= $element->getName();
          foreach ($element->getAttributes() as $name => $value) {
              $tag .= " ";
              $tag .= ($value === null) ?
                  $this->formatBooleanAttribute($name) : $this->formatAttribute($name, $value);
          }
          return $tag;
      }
docs: 開始タグまたは空要素タグの共通部分を書式化します.
  @param  Element $element 書式化対象の要素
  @return string           "<elementName ... "