When working on responsive designs, it would be very handy to have a tool like what Firefox 15 added which allows you to quickly snap to a particular viewport size and see how your code reacts to e.g. a display rotation: https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_View I also find this useful when working with the developer tools fullscreen on a laptop with limited screen real-estate as it allows you to set the view port independently of the window size, which can even be larger than the physical display.
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Now in Safari 9.