In css-masking-1:
Animation type: as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-rect">rectangle</a>
However, the definition of animation type seems to have moved from css-transitions-2 to web animations. Furthermore, the rectangle type no longer exists.
I think the correct new type is by computed value but wanted to check. @birtles @dirkschulze @tabatkins