I want my images to be dynamically sized to a percentage of the width of the window, not have hardcoded pixel sizes. And yet, I do not want this to force me into a 1994-style reflowing layout where the text moves around as the images load.
In the Bad Old Days, you fixed the bouncy-layout problem by specifying the image's width and height in pixels, but that doesn't work if you want the image to scale. In This Modern World, you have to specify "width:100%; height:auto" to accomplish that, and the "auto" means that the image shows up as zero-height until it has begun loading and its native size (and thus its aspect ratio) is known.
Basically I want some way to tell layout, "the intrinsic size of this image you're about to load is 1000px × 400px", or "the intrinsic aspect ratio of this image you're about to load is 10:4".
No. There's no sane way to fix it.
Every web site must choose between hardcoded image sizes; and having the text thrash around as the images load.
It's 1994 all over again.