The important thing with features like this is that you can't allow the user a way to disable it. If it can be disabled, the first people who disable it will b

The important thing with features like this is that you can't allow the user a way to disable it. If it can be disabled, the first people who disable it will be developers, who will proceed to continue building websites that pop this up for everybody else, oblivious that it's pissing off every one of their users. We already went through this cycle once with javascript error popups. Developers turned them off, then pasted in crappy 3rd party dhtml menu code, which in turn threw up warnings for every regular user to come down the pike. These warnings need to be prominent and painful. And they need to stay that way for everybody, otherwise they'll just be an annoyance for non-savvy users, rather than an incentive to fix the 400 million sites that are currently getting this wrong.