The Responsive Design way of thinking kicked off in around 2011. Since then, all our websites have been built with a Responsive interface and are fully tested in our device lab.
We use over 25 devices to test on and this can also include the many browsers types that can be installed on each device! that is a lot of testing!
Luckily we make use of software that can help our development and test procedures for mobiles and tablets. The likes of Ghostlab and LiveReload save us from going mad.