This is the first div.
This is the second div.
In this minimal test case the left-hand div has display:inline-table and has a blue background and a negative right margin. This allows the right-hand div, which is display:inline-table with a green background and is later in the source, to overlap the left-hand one.
The positioned z-index:auto div holder and each inline-table has stacking context–like behaviour, as does the positioned z-index:auto child within the left-hand div. However, no positioned elements have an integer z-index specified and so no actual stacking contexts exist other than that formed by the document root.
Since the left-hand blue inline-table has stacking context–like behaviour, it should be rendered atomically; in particular, no other box should be rendered between its background and its positioned, z-index:auto child. In Firefox 3 beta 5/Win, Opera 9.27/Win (buggy), Opera 9.5 beta 1/Win, Safari 3.1/Win and IE 8 beta 1 this does not happen; the background of the right-hand green inline-table is rendered between the blue background of the left-hand inline-table and its red-background positioned, z-index:auto child.
An update to CSS 2.1 Candidate Recommendation Section 9.9 (Layered presentation) validating this currently non-conformant behaviour has been approved.
View other implementation bugs related to z-index and stacking contexts.
For a discussion of the ambiguities and errors currently present in the CSS 2.1 Candidate Recommendation as regards z-index and painting order, see CSS 2.1 Candidate Recommendation : z-axis layering behaviour, stacking contexts and the z-index property.