First CSS Gradients, and now WebKit introduces us CSS Maks. Masks allow you to overlay the content of a box with a pattern that can be used to knock out portions of that box in the final display. In other words, you can clip to complex shapes based off the alpha of an image.
We have introduced new properties to provide Web designers with a lot of control over these masks and how they are applied. The new properties are analogous to the background and border-image properties that already exist.
Use of a mask results in a stacking context being created (similar to how opacity and transforms work). The mask will therefore overlay the child and all of its descendants, and at a minimum will knock out everything outside the border box of the object.