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opacity < 1.0 forms a stacking context. But should z-index apply to an element that is not positioned?
In the test case, opacity:0.66 and z-index:2 let a red box paint above a following blue box that has opacity:0.90 set. Not in Firefox 184.108.40.206.
Created an attachment (id=17505) [details]
See also: bug 11905.
Created an attachment (id=21132) [details]
Stop honoring z-index on objects that are not positioned.
(From update of attachment 21132 [details])
r=me, please add a test case (using getComputedStyle if possible, or pixel test otherwise).
Fixed in r33458.