[css-transforms] Feature request: Animate a part of an element

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Currently an animation animates a complete element. I propose transform-offset: x y w h that could animate a portion of an element.

x: Describes the X-postion of the part of the element to animate. Computed value must be between 0 and element width
y: Describes the Y-postion of the part of the element to animate. Computed value must be between 0 and element height
w: Describes the width of the part of the element to animate. Computed value must be between 0 and element width-x
h: Describes the height of the part of the element to animate. Computed value must be between 0 and element height-y