The "inset" keyword allows to draw an inner shadow in the box. To show an insert shadow, just add the inset keyword.
box-shadow: inset 10px 10px 5px #888888;
You can define as many shadows for the same box as you want by writing all of them comma-separated in the same rule.
In the example below, tow inner shadows have been created by separating each shadow with a comma.
box-shadow: inset 10px 10px 5px #888888,
inset -10px -10px 5px #888888;
In case we specify more than one value, the one which comes last will be positioned at the back of all shadows.
box-shadow: 0 0 10px 4px #FF6347,
0 0 10px 30px #FFDAB9,
30px 0 20px 30px #B0E0E6;