You have to do such testing now with JS/DOM-based resizing. The only real difference is that it's done on the server side, which simplifies things because you have only 1 rendering engine instead of say 50 client versions. And, often 2 sizes are usually fine, and 3 if you have a big audience. And it can increase the general resolution (magnification) to fill the current screen. JavaScript resizers, such as bootstrap often do similar now: you reach a threshold before they reshuffle, and in between the thresholds, they just multiply width by a constant factor to fill the screen. And bootstrap is buggy and inconsistent.
