[css3-flexbox][css-ALL] Should z-index Just Work on flex items?

I think, and I suspect others agree, that the fact that z-index doesn't Just Work
without 'position: relative' was a mistake in the design of CSS2. There's already
a distinction between 'z-index: auto' and 'z-index: 0' that can easily distinguish
a stacking context at the default level vs. a non-stacking-context at default level.

The question I have is, going forward, do we want new CSS layout modes to let
z-index do its job whenever it's non-'auto', without requiring 'position: relative'?

(If so, we should start with making it work in flexbox.)

~fantasai