<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _css_background(self, **kwargs):
 """ Get the CSS specifiers for this as a hidpi-capable background image """


        # Get the 1x and 2x renditions
        img_1x, img_2x, _ = self._get_renditions(kwargs)

        tmpl = 'background-image: url("{s1x}");'
        if img_1x != img_2x:
            image_set = 'image-set(url("{s1x}") 1x, url("{s2x}") 2x)'
            tmpl += 'background-image: {ss};background-image: -webkit-{ss};'.format(
                ss=image_set)
        return tmpl.format(s1x=img_1x, s2x=img_2x)