When using Chrome or Firefox, the z-index renders correctly, but not on Safari.
In Safari, it seems that the z-index is not taken into consideration at all. You're stumped as to what the Safari-specific problem may be.
It won't be a problem at all! Add this to the content layer and the problem will be solved by itself, thanks to this handy repair.
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#element{
  -webkit-transform: translate3d(0,0,0);   z-index: 999; }
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