This is a new flag for `<iframe sandbox="...">` which will allow a sandboxed document to spawn new windows without forcing the sandboxing flags upon them. This will allow, for example, a third-party advertisement to be safely sandboxed without forcing the same restrictions upon a landing page.
Docs: https://lists.w3.org/Archives/Public/public-whatwg-archive/2015May/0035.html https://www.w3.org/Bugs/Public/show_bug.cgi?id=28817 https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe
Samples: https://googlechrome.github.io/samples/allow-popups-to-escape-sandbox/index.html