The most common shadow is a shadow with x-offset, y-offset, blur and color.
box-shadow: 3px 3px 5px black;
There are optional values being omitted here. They are
inset allows us to create realistic button switch. It creates an illusion of button that is lower that normal level.
spread value allows us to expand or scale down the shadow effect. It is not common but we can use the
spread to create interesting effects.
In chapter 2, Drawing shapes, we will use box-shadow to create pixel art effects. In chapter 4, Skeuomorphic and Design, we will also use box-shadow to create different realistic effects.