Let's get to the heart of the matter. This peculiar behavior is caused by the default viewing priorities of the HTML elements. For example:

1. A frame will take priority over all other html elements and will be displayed on top.
2. A form element will take priority over any of the non-form elements

Below are some examples of form elements and non-form elements:

  Form Elements

• Textarea
• Select - list box or drop down
• input type text
• input type radio
• input type checkbox

  Non-Form Elements

• anchor tag (link)
• div tag
• span tag
• and table tag and all of its tributory elements (thead, tfoot, tbody, tr, td)

Among the HTML elements mentioned above are some elements which are known as windowed elements and windowless elements. Today's problem concerns windowed elements. What happens is that the operating system (by default) paints these elements on top of any of the windowless elements that may appear on a given HTML page. At this point, if you think the CSS property z-index is supposed to control the overlapping order of display for any HTML element, then you're right, it should. However, the z-index has no bearing on windowed elements in comparison with the windowless elements.

 Below are some examples of windowed and windowless elements:

  Windowed Elements

• SELECT elements.  
• <OBJECT> tag elements.  
• Plug-ins.  
• IFRAMEs in IE 5.01 and earlier.