O'Reilly logo
  • Slavomir Sobania thinks this is interesting:

Flat design is an increasingly popular trend in web design and is currently the dominant design style in mobile interfaces. Based on simplicity, minimalism, and efficiency, flat UI design eliminates much of the third dimension from the design. According to its advocates, it no longer is necessary to mimic the familiar third dimension in UI design, as people have accepted and adopted the mobile device or are practically born with it in their hands and don't need the third dimension anymore. The mobile device is now ubiquitous and can stand on its own.

From

Cover of Web Design Blueprints

Note

3rd dimension is present even in layout of text. It's part of human perception.