After each deployment at work, I opened and tested the pages on IE tester. An application that could run several versions of Internet Explorer. Then I'd open the pages on my HTC Evo mobile phone. Other devs will test on their own respective device. We made sure it worked everywhere. It was a pain.
In fact, I worked with several companies that have no way of knowing how their website is performing. They don't check which browser their customer uses, and they don't track performance in any way.
Is this it? Did devices evolve in a way that we don't need to check on them anymore? Maybe so, but I'm still skeptical.