For basic masking (square) you can set the overflow to hidden on the parent object.
For webkit mask, you could add something like this to the custom css property for an object base state. You'll find the 'custom css' property under the 'Other' property group in the property panel.
Here is some Apple/Safari documentation on webkit masking
-webkit-mask-image: -webkit-gradient(linear, left top, left bottom, from(rgba(0,0,0,1)), to(rgba(0,0,0,0)));