@classmethod
def tag_start(cls, tag, attrs=None):
    """Build started tag info.

        :param tag: tag name
        :param attrs: tag attributes. Default：``None``.
        :type attrs: dict or None
        :rtype: str
        """
    if attrs:
        return '<{} {}>'.format(tag, cls.build_attr(attrs))
    return '<{}>'.format(tag)
