how to set options to get mobile CSS only?

Hi, team,

I am trying to use UnCSS to generate mobile version CSS for a page. How could I achieve this?

I tried setting the userAgent to "mobile" or "Android" or "iPhone" or other strings from mobile devices, also tried setting with the "media" options with value such as "(max-width: 767px)" etc, but the results I got are always the same as the desktop version CSS.

Could you please give some hints or directions on how to achieve this?

Thanks, Abe