Design for Mobile First
When creating a web app or website, designers often embrace responsive design in order for users to be able to use layouts on a variety of device screen sizes. A design created for mobile devices is more likely to work on desktop without special consideration than the other way around (a desktop design on mobile).
Not only that, but the constraints of mobile UI force designers to prioritize and cut the fluff. Generally the results are a simpler experience to accomplish the core objectives of what the app sets off to do. This is not to say both the mobile and desktop versions should be perfect copies of each other, the point is starting with mobile design iterations drives you to simplify the complex from the start.