led.css is a CSS class that allows you to easily create LEDs in pure CSS.
The default LED can be created like this:
<div class="led"></div>
The default LED is in an "off" state, so add some color:
<div class="led led-red"></div>
<div class="led led-orange"></div>
<div class="led led-yellow"></div>
<div class="led led-green"></div>
<div class="led led-blue"></div>