In Windows 8, there are two “modes” of use: Metro mode and classic mode. Metro mode sports the spiffy new UI while classic is the same old boring Windows of yore. When you run Internet Explorer 10 in Metro mode (the default) there’s a cool new feature that lets you “snap” a window to the side so you can use two simultaneously. This window, of course, is made to be far more narrow.
Tim Kadlec beautifully explains how to get responsive layouts looking good in Windows 8’s Snap Mode.