[geometry] Why does DOMMatrix not skew or flip on the Z axis?

DOMMatrix classes only skew on X and Y, but not Z. Why not?

(I also noticed that CSSSkew in Houdini also ignores the Z axis).

On a side note, I'd like to update my DOMMatrix{ReadOnly,} polyfills to
editor's draft soon. http://github.com/trusktr/geometry-interfaces

*/#!/*JoePea