Skeleton's base grid is a variation of the 960 grid system. The syntax is simple and it's effective cross browser, but the awesome part is that it also has the flexibility to go mobile like a champ. Go ahead, resize the browser and watch as the layout reacts!
Skeleton uses a lot of media queries to serve the scalable grid, but also for the convenience of styling your site on different size screens. Skeleton's media queries are almost exclusively targeted at max and min widths rather than device sizes or orientations. The advantage of this is browsers and future mobile devices that don't map to exact set dimensions will still benefit from the styles. That being said, all of the queries were written to be optimal on Apple iOS devices. The built in media queries include: