screen.html uses data from http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density to simulate mobile device screens. For each screen, it creates an iframe with the specified width and height and then uses -moz-transform (it is Firefox-specific, for now) to scale that iframe to simulate the pixel density of the mobile screen. There is an input field for entring the pixel density of the screen you're running on. It defaults to 113 dpi, which is the density of the 13" MacBook Pro. There is also an input field for specifying what URL to display on the simulated screens. It defaults to testpattern.html which is an attempt at a flexible layout based on media queries and the CSS rem unit.