To whom it may concern: CSS has an aspect-ratio property that can be used to set/suggest an aspect ratio for images, iframes, etc. that you only set one dimension on (e.g. width: 100%), and in HTML img has a loading="lazy" attribute which makes an image only load when it is scrolled to (iframe also has this attribute).
(lhfm made this post after looking at my photo galleries which take a lifetime to load even with the images resized and compressed)