Masks allow you to obscure areas of a box element by overlaying it with an alpha mask.
Examples
An example of using -webkit-mask property, to clip the image to create a fuzzy edge effect.
photo
This is the alpha mask image (png-24) applied to created the effect above.
Another example of the same image with a different mask.
photo
photo
#photo { -webkit-mask-box-image: url(images/mask.png); }