Flat Design Elements [Buttons,Accordion,Tabs,Navigation,Login Form]

Flat Design,a popular user interface and to have an idea how does the flat design looks like,I've written a bit of code to show you the live demo through a pen.So have a look at the pen below.

If the pen is not visible correctly check this.

See the Pen Flat Design Elements by Mohammad Hamza Dhamiya (@hamzadhamiya) on CodePen