[tedunangst]

Are you also going to download every resource listed in every @media section of the CSS regardless of screen size?

[blotter_paper]

That part doesn't seem too unreasonable to me, but you could also just go with the largest available size and then scale it as necessary on the client.

[missblit]

Scaling down the largest size isn't always appropriate (though would probably work in most cases).

One example might be a set of images where the smaller images wrap text more agressively to work better on a screen that's not as wide.

[blotter_paper]

Good point, I've seen comparable use cases in the wild but it slipped my mind!

[oftenwrong]

The browser could pick a fake screen size, and behave in a way that is consistent with that fake screen size. This would probably break many sites, but it would mitigate fingerprinting if a common size was used.

[pmoriarty]

This solution is just begging for side-channel attacks.

Firefox had better make sure its timing is not affected by such shenanigans, for instance.

[oftenwrong]

I doubt that is avoidable, as the browser would still probably need to render at the false viewport dimensions. For a common adversary, fingerprinting based on timing would be more involved and less useful.