Question on z-index on IE8 browser

Using Developer toolbar on IE8 , on one page i see that the z-index for a DIV shows as "auto", I went from this element to its parent hierarchy checking if some element has actually defined this attribute(z index) but I did not see any element that has the css style attribute z-index defined/set.

I created a sample page where I did the same thing and it shows me the z-index as 0.

I have not defined my div or any other element to have zindex attribute but still it shows me the z-index as 0.

I am trying to find out why would I get the ZIndex as 0 on one page and Z-Index as "auto" on other when it has not been defined in eit开发者_如何学JAVAher of the pages. Here is the sample of the css for that div.

.mydiv
{
position:absolute;
overflow-x:hidden;
overflow-y:auto;
}

thanks

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I'm guessing here but since position: relative/absolute triggers a new z-index stacking context in IE < v9 I suspect that's the distinction.