Flat Design is my style of choice when it comes to HTML5/CSS3 because it places functionality over extra effects. As a minimalist approach to design, the focus is on visual clarity as opposed to elements that may add realism, but don’t contribute overall to the user experience. With flat design, there is less shadowing, gradients, and 3D. Instead, emphasis is placed more on color schemes, typography, and shapes. A popular example of flat design would be the Windows 8 User Interface
Here is a small web app I found that helps designers choose color schemes right from their phones to get a feel for Flat Design UI. Below, I handpicked the following articles to help new users understand the flat design trend. A few available UI toolkits will also be listed in Our Code Wiki for you all to access later.
Flat Pixels: A Battle Between Flat Design and Skeumorphism by Sacha Greif
A Look at Flat Design and Why It’s Significant by Luke Clum
The Flattening of Design by Nick Bilton
Flat and Thin are In by Adrian Taylor
12 Creative Examples of Flat Design by CreativeMarket.com