Consider the text in https://drafts.fxtf.org/geometry/#dom-dommatrix-multiplyself:

The otherObject matrix gets post-multiplied to the current matrix.

This is not linking to https://drafts.fxtf.org/geometry/#post-multiply but presumably means to? But the way that's meant to be used is "X post-multiplied by Y". Whereas the language here is "X gets post-multiplied to Y". Does that mean "X post-multiplied by Y" or "Y post-multiplied by X"? It's entirely unclear to me.

There's also various usage of "post-multiplie X on Y" (e.g. in https://drafts.fxtf.org/geometry/#dom-dommatrix-translateself) and so forth.

There's just no way to implement this without reverse-engineering browser engines, as far as I can tell.

@dirkschulze @chrishtr @zcorpan