Came across a couple of issues while implementing CSSTranslate, but they apply to others:

Many of these attributes' behaviour (for example the x/y/z attributes of CSSTranslate), are not specced at all.

Second, the spec for the x/y/z attributes on CSSRotate and CSSScale say:

The x, y, and z attributes must, on setting to a new value val, rectify a numberish value from val and set the corresponding internal slot to the result of that.

The WPT tests (eg https://wpt.live/css/css-typed-om/stylevalue-subclasses/cssTranslate.tentative.html) expect these to throw a TypeError if they are set to a value that does not match the expected numeric type, similar to the errors thrown in the constructor. This makes sense, but isn't mentioned in the spec.

EDIT: Removed what I said above about rectifying being a no-op, I didn't notice that CSSRotate's attributes are indeed CSSNumberish.