In iOS there's a bug that prevents the webcam video feed from displaying when a website is added to the home screen and presents itself as web app capable. That is, when the <meta name="apple-mobile-web-app-capable" content="yes"> tag is present, the video feed refuses to appear.
Working example here. Broken example here. After visiting each page, choose "Add to Home Screen" from Safari's share button then open the web app from iOS' home screen.
Tested on iOS 12 public beta 3.