Why Follow Standards?
Simple: so you don’t have to look like a fool and publish gibberish like this.
“Because they are windowless, iframe elements support the zIndex attribute in Internet Explorer 5.5 and later. Windowed controls, such as select elements, ignore the zIndex attribute. If your applications were designed for earlier versions of Internet Explorer, you might want to redesign pages containing iframe elements that are stacked on top of windowed controls. You can use the visibility attribute to hide windowed controls that you want an iframe element to overlap. You can also position windowed controls so that iframe elements do not overlap them.”
… Or Internet Explorer could follow the rules, and web designers wouldn’t have to worry about Select boxes appearing on top of divs, and other such crap that makes IE a pain to make webpages for.