Append a link to this container.

        :param child: a string indicating the location of the linked
            document
        :param rel: Specifies the relationship between the document
            and the linked document. If not given ``stylesheet`` is used.
        :param type: Specifies the content type of the linked document.
            If not given ``text/css`` is used. It an empty string is given,
            it won't be added.
        :param media: Specifies on what device the linked document will be
            displayed. If not given or ``all``, the media is for all devices.
        :param kwargs: additional attributes

def insert(self, index, child, rel=None, type=None, media=None,
               condition=None, **kwargs):
        '''Append a link to this container.

        :param child: a string indicating the location of the linked
            document
        :param rel: Specifies the relationship between the document
            and the linked document. If not given ``stylesheet`` is used.
        :param type: Specifies the content type of the linked document.
            If not given ``text/css`` is used. It an empty string is given,
            it won't be added.
        :param media: Specifies on what device the linked document will be
            displayed. If not given or ``all``, the media is for all devices.
        :param kwargs: additional attributes
        '''
        if child:
            srel = 'stylesheet'
            stype = 'text/css'
            minify = rel in (None, srel) and type in (None, stype)
            path = self.absolute_path(child, minify=minify)
            if path.endswith('.css'):
                rel = rel or srel
                type = type or stype
            value = Html('link', href=path, rel=rel, **kwargs)
            if type:
                value.attr('type', type)
            if media not in (None, 'all'):
                value.attr('media', media)
            if condition:
                value = Html(None, '<!--[if %s]>\n' % condition,
                             value, '<![endif]-->\n')
            value = value.to_string()
            if value not in self.children:
                if index is None:
                    self.children.append(value)
                else:
                    self.children.insert(index, value)