Considering that web application users use a wide variety of web browsers, web programmers must run compatibility and performance tests on various browsers before an app goes live. As a result, you'll have to install all the popular web browsers on the same computer, and you'll even have to run the same tests on all kinds of media devices (such like PC, smart phones and tablets) to simulate how an application works under different page sizes, resolutions and so on conditions. That is really head-scratching and very inefficient.