No it's not. Look at the address bar. He injected that image tag with a value that would cause an error (the x value as opposed to a valid image url) which triggers the onerror javascript to execute. In this case it prompts the user but it could have been anything really.

I think you completely missed my point. I am simply pointing out prompt()is provided by Apple just like alert() provided by JS itself.

Before coming to a conclusion that you can point to and say someone is wrong, please understand that not everyone here is a master of English.

prompt() is a built in JavaScript function just like alert(). It isn't Apple specific, that's my point.