Scroll down to see some examples.

.no-js-testimg_1d61597ba3 { display: none !important; }

.no-js-testimg2_8abb655648 { display: none !important; }

This image has "proportionalHeight={100}", so its height is equal to its width.

.no-js-testimg_1d61597ba3 { display: none !important; }

This image has just a plain url, so it does neither show a placeholder nor use webp oder srcset.