Previously, I would use an iframe to reference an external document. Now I'm using flexbox and I can't find any information on how to fill it with the contents of an external document, similar to the way that iFrame would have worked, except that it would resize dynamically the way that FlexBox does. I have tried converting the HTML into a string and inserting it with JavaScript, but the problem is that the file needs to be an interactive app, not static data. Basically, I want to have a box on my website that will resize as the browser window resizes, and that will point to an app that will function inside that box. What's the best way to do this?