Created attachment 197150 [details]
Open the attached file rotate-perspective-webkit-bug.html in a browser that supports regions (Chrome Canary or WebKit Nightly) on Wino or Mac.
The green polygon looks like it has skew applied to it, instead of rotateX with perspective 3D transform.
The green polygon looks like it has rotateX with perspective 3D transform. The top edge should be shorter than the bottom edge and the left and right sides should be slanting inward. The shape (but not the position) should look like rotate-perspective-webkit-bug-no-region.html
Created attachment 197151 [details]
expected results (no regions)
After some more investigations it looks like content flowed in a region that has a 3D transform applied does not take into account the perspective (whether specified via -webkit-perspective on a parent or via the perspective() transform function in -webkit-transform).
Still repros on today's nightly (r151543)