This is without a doubt the most significant recent change. In a way, it is the mother of all changes. I certainly hope you have no doubt about the importance of mobile devices in the work of a web developer.
At the time of this writing, average mobile device usage, when it comes to big websites, is about 15% here in the Czech Republic. However, worldwide, usage is closer to 50%. I base this figure on publicly available statistics, but take a look at your own website stats.
In the rest of the world, web developers must expect the share of mobile devices to be over 50%.
You can’t really hide from mobiles.