Checking out the "intrinsicsize" spec and it seems really reasonable. It allows you to specify the intrinsic width/height for an image/video element; no need to wait for it to download and thus potentially cause janky page reflows. https://github.com/ojanvafai/intrinsicsize-attribute
"Why just image and video? These are the only elements in [the HTML spec] that size to an external resource." <-- I always found this weird. An <img> has to download the PNG/JPEG before it can figure out how big it should be!