<iframe> is a valid tag for all transitional and frameset DOCTYPE's.
There are three flavours of (X)HTML, strict, transitional and frameset.
Strict has no depreciated tags in it at all.
Transitional has all of the depreciated tags in it such as <iframe> and <font> except for <frameset>, <frame> and <noframes>.
Frameset has the tags in it that Transitional doesn't.
<iframe>'s are now completeley gone from the latest standard of XHTML 1.1 (there is only a strict version of this), so by using <iframe> you are using old outdated (but not IE proprietary) code. With the correct DTD it's still valid.
I only included styles into the latest script because it highlights each level of the dropdown. The only styles that are necessary are the inline styels in the div tag (style="display:none;").