Iframe glitch when displaying HTML in Firefox

I’ve been trying to adapt the YouTube iframe feature to display HTML pages instead. It seems to work fine in most browses except Firefox (which seems to encounter a few Playcanvas bugs and performance issues, though it seems to be improving gradually). An artifact occurs at certain angles, covering part of the image with black. A YouTube embed or direct MP4 URL works alright. It’s just HTML that runs into trouble.

Displaying HTML page in Firefox:

Displaying embedded YouTube video in Firefox: