The 'mask-image' property has the new optional keywords 'luminance' and 'alpha' to differ between the masking type. Without the argument, or with 'alpha', the transparency level is chosen. For 'luminance' the luminance value is chosen. This is backwards compatible with WebKit's definition of '-webkit-mask-image'.
Ahh, already opened a bug report :P. *** This bug has been marked as a duplicate of bug 97194 ***
*** This bug has been marked as a duplicate of bug 97134 ***