From http://view.jquerymobile.com/1.3.2/dist/demos/intro/rwd.html

mobile and up

/* Start with core styles outside of a media query that apply to mobile and up */
/* Global typography and design elements, stacked containers */
body { font-family: Helvetica, san-serif; }
H1 { color: green; }
a:link { color:purple; }

/* Stack the two content containers */
.main,
.sidebar { display:block; width:100%; }

/* First breakpoint at 576px */
/* Inherits mobile styles, but floats containers to make columns */
@media all and (min-width: 36em){
	.main { float: left; width:60%; }
	.sidebar { float: left; width:40%; }
}

/* Second breakpoint at 800px */
/* Adjusts column proportions, tweaks base H1 */
@media all and (min-width: 50em){
	.main { width:70%; }
	.sidebar { width:30%; }

	/* You can also tweak any other styles in a breakpoint */
	H1 { color: blue; font-size:1.2em }
}