It seems getOMTAStyle() returns an empty string in delay phase (even though we send the animations into the compositor) [1]. It'd be better to return a valid string to know its status.

Note: In this case, the SampleResult is None.

[1] https://searchfox.org/mozilla-central/rev/153172de0c5bfca31ef861bd8fc0995f44cada6a/dom/base/nsDOMWindowUtils.cpp#3424

I just happened to remember there is an API to get transform values on the compositor, its name is getOMTCTransform, it might be sufficient for the test case in question.

Oh yes. I got a matrix like this matrix(1.25, 0, 0, 1.25, 87.5, -12.5). It seems there is a -1.25 offset on the translate part (note: what I expected is matrix(1.25, 0, 0, 1.25, 100, 0)). I haven't checked the code yet, but this works in delay phase (though it's a little bit different from the expected matrix).