Turns out any <model-viewer> embedded in an <iframe> gets the capped DPR=1 (no high-res render) unless the iframe has a meta viewport tag in its own <head>. That's a bit weird, since meta viewport tags are ignored in iframes and only the top-level document counts. As such, we'll assume the top level document has a meta viewport tag (as that's more common) when the element is inside an iframe and can't query its parent's <head>.
Turns out any
<model-viewer>embedded in an<iframe>gets the capped DPR=1 (no high-res render) unless the iframe has a meta viewport tag in its own<head>. That's a bit weird, since meta viewport tags are ignored in iframes and only the top-level document counts. As such, we'll assume the top level document has a meta viewport tag (as that's more common) when the element is inside an iframe and can't query its parent's<head>.