In HTML5, Z-depth is pretty exclusively used as a mechanism to
determine Z-order in the HTML / CSS separation. Normally, the order of
HTML determines the Z-order. But, Z-order is really a style thing,
rather than a composition thing. So, specifying the Z-depth allows you
to override standard behavior in the style sheet. This is not
necessary in a true object-based GUI where graphical objects are more
self contained and can easily determine their own Z-order.
I doubt we will see general purpose adoption of any 2D interface that
will rely on Z-depth for anything besides determining Z-order. For
instance, the example of shadow size seems interesting but runs into
usability problems quickly. Let's say that you have element A with a
larger shadow to indicate that it is above element B with a smaller
shadow. That seems great. What happens if element B is selected and
brought to the forefront? Does A's shadow go down in size? If so, that
breaks the normal shadow analogy. If not, it would seem that the
shadows would just keep growing bigger each time a new element is
brought to the front. For limited general-purpose applications (e.g.,
a larger shadow for an object currently being dragged), you don't need
Z-depth info. For specialized applications where a Z-depth might make
sense (I can't even think of one), there's little problem adding such
functionality to custom widgets. So, my recommendations is to stay
away from trying to implement Z-depth as a fundamental component of a
2D interface. The benefit is minimal and largely theoretical.
+ 1
Stef
