A SWF file in a layer on a DHTML page containing several layers displays above all the layers regardless of the layers' stacking order ("z-index").
Solution: Use the WMODE parameter
Use the WMODE parameter to allow layering of rich media content with DHTML layers. The WMODE parameter can be "window" (default), "opaque," or "transparent." Using a WMODE value of opaque or transparent prevents a SWF file from playing in the topmost layer. And, it allows you to adjust the layering of the movie within other layers of the HTML document.
For details on creating SWF files with transparent backgrounds and more information on the WMODE parameter, see Create transparent backgrounds in a SWF file.
Note: The WMODE parameter is supported only on some browser/Flash Player version combinations. If the WMODE parameter is not supported, the SWF files always display on top.
By default, browsers place embedded plug-in content, such as a SWF file or Java applet, on the topmost layer. In older browsers, attempts to place a DHTML layer on top of a Flash layer fail. Newer browsers add the ability to layer rich media content with DHTML content. And, in some cases, they add the ability to use transparent backgrounds in the rich media content.