[cssom] "Mutating the declarations" is underdefined

[smaug----]

Is the latter of the following a "mutating the declarations"
element.style.width = "10px"; element.style.width = "10px"; ?

The reason why this must be defined is to decide what kind of MutationRecords should be created when style attribute is set.

Given that element.setAttribute("style", "width: 10px;"); element.setAttribute("style", "width: 10px;");
creates two MutationRecords, I think for consistency also element.style.width = "10px"; element.style.width = "10px"; should. And that is the behavior in Gecko and Edge (but not in blink)