Chapter #7. Make Your Buttons Look Like Buttons
The flat design aesthetic, born out of Microsoft's Metro user interface, rose to near ubiquity in the late 2000s. In iOS 7 and Android's material design, these extremely minimal visuals are still the go-to look for modern web applications.
Flat design is bad. It's really terrible for usability. It's style over substance and it forces your users to think more about every interaction they make with your product. Stop making it hard for your customers to find the button:
There are parts of your UI that can be interacted with, but your user neither knows which parts these a, nor wants to spend time learning this. They have used buttons in real life, many times—on elevator controls, on their oven, and in their car—so they understand how a button works: