Use the browser's onresize event to work out what changed and modify the width and height attributes accordingly. Alternatively, design with lazy loading in mind and use screen relative units (eg vw, vh, vmin and vmax).
Screen relative units are not possible to be used here - you have no idea how big the image will be on the screen, since you don't know how big the screen is.
Using js onresize event is a no-go, if JS is still needed the spec is worthless. Lazy loading images with JS worked before just fine, this has to be about not needing special JS for this feature.
No, parent is right: It's impossible to satisfy that recommendation, usually.
Screen relative units are not possible to be used here - you have no idea how big the image will be on the screen, since you don't know how big the screen is.
Screen relative units represent a percentage of the screen size (technically speaking it's the viewport really). You don't need to know how big the screen is; you're tell the browser to use, for example, 25% of the viewport height when you set something to be 25vh. If you want 4 images vertically that's really useful. That way the browser knows it needs to put a box 25% of the screen height in the DOM where the image will be loaded later so there's no layout thrashing.
Using js onresize event is a no-go, if JS is still needed the spec is worthless.
The parent suggested they would want to change the width and height attributes of the image when the user resizes their browser. That's exactly what the resize event is for. If you want to display a fixed size image you can use fixed width and height attributes. If you want the image to scale with the viewport size you can either use units that represent a relative size or you need to use JS to modify the attributes. Heck, you can even use media queries to mix and match and have fixed size in some devices and relative in others.
Lazy loading images with JS worked before just fine..
Except you needed JS to do it. With the new HTML spec you get the benefits of basic lazy loading without needing the additional weight of some JS code. That's good.
But you never know that an image should cover 25vh! That depends on the size of the viewport in total, the resolution and the specifics of the layout situation, things like text size. And almost always the width of the viewport decides image width and thus the height of the image.
This totally misses the point. I can't run all those calculations while rendering a website. The browser needs to be intelligent enough to not dynamically jump around in the viewport when an element finished loading. Just fix the viewport and attach at the top and bottom.
And relative units don't fix this either as that information still misses the ratio of my image. If it's 100 vw, how high is it? I don't know this information without fetching the image, doing calculations and dynamically updating the DOM with that information.
That sounds like a problem that should be fixed in CSS. Ideally, it should be possible to specify that an image should have a width and/or max-width of 100vw, 100% of its parent, or whatever, taking box-sizing and paddings into account... and then specify that its height should be a certain percentage of its own width. The server already knows the aspect ratio, so it can deliver this information in a 'style' attribute.
I haven't been following the latest developments in CSS for a while, so maybe it's already possible to do this?
I do this in production a few places by setting a div with no content or only absolutely position content to have padding-bottom of the aspect ratio in %. That gets calculated from the width, so if the image is 4:3 my ratio is 75%; 200px wide becomes 150px tall as well. Then, I set the image as a background to the div or absolutely position it inside to fill the space, and it's responsive to the layout while also reserving space for itself.
