code: def tag_start(self, tag, attrs=None):
          r
          return '<%s %s>' % (tag, self.build_attr(attrs)) if attrs else '<%s>' % tag
docs: r"""Build started tag info.
  
          :param tag: tag name
          :param attrs: tag attributes. Default：``None``.
          :type attrs: dict or None
          :rtype: str