[geometry-1] Why does multiplySelf (etc) accept DOMMatrixInit instead of DOMMatrix?

[trusktr]

I don't see why multiplySelf should accept a DOMMatrix-like object, rather than an actual DOMMatrix object.

If multiplySelf accepted DOMMatrix instances only (like an API of this sort would typically do, where "other" in a multiply(other) type of method is typically the same type of object as the object on which the method is called), then there would be no need to do runtime type checking of plain JS objects, which would improve performance.

By making it accept DOMMatrix instances, it would be possible to avoid the fixup steps, because the operations on DOMMatrix always produce valid properties (f.e. a is always the same value as m11).

instead of this:

const a = {m11: 1, ..., m44: 1}

// later
const b = new DOMMatrix().multiplySelf(a)

we could have

const a = new DOMMatrix([1, ..., 1])

// later
const b = new DOMMatrix().multiplySelf(a)

Is there a specific reason why we want to pass in non-DOMMatrix objects?