Folks in Google's anti-malvertising team would like to be able to prevent sandboxed frames from popping up confusing, modal messages to users. This includes things like `alert()`, `confirm()`, and `prompt()` (and `print()` (and maybe authentication dialogs)).
Docs: https://lists.w3.org/Archives/Public/public-whatwg-archive/2015May/0035.html https://www.w3.org/Bugs/Public/show_bug.cgi?id=28818 https://developer.mozilla.org/en-US/docs/Web/API/Window/alert https://developer.mozilla.org/en-US/docs/Web/API/Window/confirm https://developer.mozilla.org/en-US/docs/Web/API/Window/prompt https://developer.mozilla.org/en-US/docs/Web/API/Window/print
Samples: https://googlechrome.github.io/samples/block-modal-dialogs-sandboxed-iframe/index.html