Generate responsive breakpoints.
/* Mobile-first breakpoints */
@media (min-width: 640px) { /* sm */ }
@media (min-width: 768px) { /* md */ }
@media (min-width: 1024px) { /* lg */ }
@media (min-width: 1280px) { /* xl */ }
/* Max-width */
@media (max-width: 639px) { /* mobile only */ }