The zIndex that you assign to different elements in your web page is supposed to determine which elements appear in front when the position of two elements overlap. There are a number of instances where no matter what you assign as the zIndex a particular element always ends up appearing in front of the other. ere we'll look at when this can occur and how we can fix it (where it is fixable).
<param name="wmode" value="transparent">
Fortunately in the case of the select and iframe problems the most recent versions of the affected browsers have resolved the issue and will use the appropriate zIndex to determine what should appear in front. This problem will therefore gradually disappear as people upgrade their browser.
This article written by Stephen Chapman, Felgall Pty Ltd.