Testing your website before launch is always important, but when it comes to websites built responsively the complexity of testing in compounded. When testing your responsive site before launch you should test across all of the major devices, and browsers, but also look at tools like Ish that will help you view random viewports (you never know what the next popular viewport will be). Device testing is always the preferred option, and there are more and more device testing labs popping up around the world. When devices are not an option there are other approaches you can use like emulators or getting a subscription to Browser Stack. When deciding on you base test devices/viewports you should make your decisions based on your existing website traffic.
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