That should have been obvious. Glowforge does all its heavy lifting for focus and positioning by super-duper optics and image processing. But I would try and engrave something on mylar. Well, you can’t. At least not on shiny 7-mil mylar with a bit of a curl in it.
Ok, you can. If you put on masking tape and hold it down really flat with lots of magnets. Flat, because a few mils in or out of focus is the difference between engraving and burning all the way through. Masking tape because otherwise the camera and the focusing system don’t see the mylar, they see whatever part of the inside of the case is reflected into their field of view by the mylar. And hitting the print button leads to that dreaded load error(possibly true for other highly reflective surfaces as well).
And about those magnets: you put a fold of packing tape around them to give you something to grip when pulling them off the bed? Nice, thin, bendy, shiny packing tape.