Custom CSS bug with -webkit-mask-image

Hello,
Just a note to point out Custom CSS is stripping out this webkit CSS3 property

-webkit-mask-image

when used like this:

.div {
-webkit-mask-image: -webkit-gradient(linear, left 90%, left bottom, from(rgba(0,0,0,1)), to(rgba(0,0,0,0)));
}

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