https://drafts.fxtf.org/geometry/#dom-dommatrix-scale3dself

Currently scale3dSelf() is described with 5 steps, but maybe we could do just do two steps for better readability and less duplication?

• Apply a scaleSelf() transformation to the current matrix with the arguments scale, scale, scale, originX, originY, originZ.
• Return the current matrix.

(cc @bzbarsky)