I would normally go with
max-width in practice. I was just experimenting with it here, with the suggestion here there was a problem with it.
Yes, aspect ratio is fairly redundant of an image, since it’s not difficult to keep an image’s aspect with ‘regular’ CSS. Again just throwing things into the experiment, maybe without fully thinking it though.
The iframe is a more plausible use case.
This is what I initially did:-
But the content did not display properly in the frame, like the content was larger and overflowing. I saw the same result with both the 360 image embed and a Youtube video embed.
Changing to width and height at 100% fixed it.