how to know iframe is loaded in js
???
https://codepen.io/xgqfrms/full/oOEbvQ
iframe & HTTPS & CORS
https://iframe.xgqfrms.xyz/eapp/index.html#blog.sina.cn
how to know a url is loaded in iframe
window.location !== window.parent.location
https://tommcfarlin.com/check-if-a-page-is-in-an-iframe/
https://www.codeproject.com/Questions/213344/Find-current-url-loaded-into-iFrame-inside-html-pa