If you're seeking a way to create a drop shadow effect using CSS3, the attribute you need is box-shadow. The format typically looks like this:
box-shadow: h-shadow v-shadow blur spread color inset;
While creating a gradient drop shadow like the one shown in the image may not be achievable with current CSS capabilities, a similar shadow can be achieved with something along these lines:
box-shadow: -10px -10px 20px #000;
You'll have to experiment with the values to perfect the look you desire, but this should serve as a good starting point for your project.