I've read it a few times, and I think that the definition of Accumulated 3D Transformation Matrix has the exit condition of the loop (step 4) stated incorrectly. I've read it a bunch of times... but I'd really like some other folks (cc @mattwoodrow @smfr) to double-check this, though!

It currently says:

While current element is not the element that establishes the transformed element’s 3D rendering context:

However, I believe the loop should execute when current element is the element that establishes the 3D rendering context, and then the loop should terminate after that iteration. In other words, I think this should instead say:

While current element is an element in the transformed element's 3D rendering context:

I've written a testcase that is interoperable across Chrome, Firefox, and Safari, and that I think matches what I'm proposing.

But, again, I'd really like others to confirm that I'm not misreading/misthinking this somehow!