In Internet Explorer 5 and earlier, iframes could not play in the world of z-index-positioned elements. While you could position an IFRAME on a page, you could not lay content from the host document over the iframe using a greater z-index value. In Internet Explorer 5.5, this feature is now supported. Internet Explorer version 5.5 also supports transparent frames. By declaring the background of the frame's document to be transparent and having the container declare that it allows transparency, you can enable the background of the containing document to show through.
By specifying the z-order of floating frames, you can stack one frame on top of another.Test