The environment in which your application is being used, has an influence on the display, behavior and speed.
Behavior of humans and software will differ based on the environment in which they operate.
Mobile devices pose more challenges with various models, screen sizes and bandwidths.
Test the application across various PC/mobile browsersTest the application on different platforms such as Windows, Unix, LinuxTest the application on different versions of web/app serversTest the application with different set of screen resolutions and bandwidths
Ensure the apps works on a variety of end user environmentsAutomate tests to reduce mundane laborMonitor the real user environments and optimize testing environments accordingly