That's just not realistic. In addition to the reasons given in the other replies, there is a long-standing Firefox bug which swaps the contents of iframes under some circumstances. It happens with iframed ads frequently.

If you have to code for IE6 you can't use transparent PNGs can you?

You can -- it's just inconvenient.

Actually, it's not necessarily using document.write() which is causing the problem. The problem is that most browsers will not move on until the script has been loaded and run, because of things like document.write(). My point is that not using document.write() won't fix the problem.

Using javascript to create script nodes can get around this problem, but only if the script you're loading doesn't have (for example) document.write(). So it's useless for ads as they are currently done.

I've actually tried overriding document.write(), and got it working fairly smoothly in FF. Not in IE, of course.

Or, if you're going to go the route of ringtones, make them really annoying ringtones, like I do.

Achy Breaky Heart. Hold On. Islands in the Stream.

I could go on.

A gas tank is incapable of getting you down the street, therefore your car is incapable of getting you down the street?

Fallacy of Composition

An arbitrary minimum age is probably unfair, but it's much less prone to abuse than an I.Q. test (or something similar).