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The working draft says in the prose: A 3D rendering context is established by a a transformable element whose computed value for transform-style is preserve-3d, and which itself is not part of a 3D rendering context. Note that such an element is always a containing block. but we don't explicitly call this out under the transform-style property. WebKit/Blink have this behavior because of a bug. Firefox seems to have copied them or gone by the spec prose. Not sure what IE does. The new spec text doesn't require that preserve-3d create containing block. I don't think it should.
Created attachment 1557 [details] Testcase
IE 11 does not treat it as containing block.
Mailing list consensus was that it should: https://lists.w3.org/Archives/Public/www-style/2015Jan/0545.html
The working draft now says "A value of "preserve-3d" establishes a stacking context, and a containing block.".