That is, if you do something like this inside the iframe
you would get an exception in Firefox, and IE (at least the more recent versions)
I guess that is what the RFC may have prescribed. However, the behavior in Chrome and Safari is different. (And chrome and safari both are based on Webkit, so its essentially webkit). They will not throw an exception, but instead merely return undefined.
Now this initially seems counter intuitive, but to think of it, I believe there is a reason why webkit behaves so. Let's say you have the following code
will find that their code wouldn't work as expected on Safari and Chrome.
Which is why it is important to adhere to the specs, in letter and spirit.