This tutorial uses the popular fontawesome iconfont to create animated checkbox inputs using CSS3 transitions. The nice thing is that one doesn't have to create custom graphics in Photoshop for it.
Pseudo elements are used on labels to place two icons(checked and unchecked) one above the other using absolute positioning. When the user clicks on the label, the checked icon animates in/out.
To simulate real handwritten ticking/unticking, the checked icon's visibility is revealed from left to right using max-width transition.