Mark Simpson (of Boostly) explains in a short YouTube clip that websites should be designed mobile-first because, over the past five years, traffic has shifted from desktop/laptop to mobile phones; Boostly builds websites mobile-first for this reason.
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One of the key indicators that you need to look at is the ratio of people that go onto your website on a desktop or a laptop computer to the ones that are going on a phone. Now, what we've noticed over the last five years is that the used to tip very heavily in laptops and computers. But now over time, it started to be this. Your website has to be has to be built mobile first, which is why every boostly website is always built mobile first. What does that mean? Is that they are designed with these things in mind because we know that the majority of your future potential guests are going to be accessing your website via a mobile phone first.