https://html.spec.whatwg.org/multipage/embedded-content.html#process-the-iframe-attributes In particular, this load must cause contentWindow to be non-null. https://hsivonen.fi/about-blank/ is good reading.
https://html.spec.whatwg.org/multipage/embedded-content.html#process-the-iframe-attributes
In particular, this load must cause contentWindow to be non-null. https://hsivonen.fi/about-blank/ is good reading.