Split/multi window views for viewing differences between mobile and desktop
When working on responsive designs, it would be fantastic to be able to view two designs simultaneously, to get a better idea of how different elements change from one breakpoints to the next. Is the padding on these two elements the same? Is this font size the same? While it's possible to check now by switching back and forth, the switching is so disruptive and tedious to said task that it's difficult to get a good idea of the overall changes, and more convenient to try to catch them as they're implemented.