In reading responses to my initial post I noticed a bit of confusion regarding the proposal. In particular people seemed to think I was proposing some sort of mandatory changes (e.g., fix your Web site or it will break!). That is not what I was saying.
Designing for higher DPI screens is about making your Web site look better when presented in the same amount of physical space on a screen with more pixels. It is purely optional. If you do nothing at all, your Web site will still look essentially the same even when magnified (assuming your system has done a good job with scaling).
Think about TVs for a second instead of computers. In particular, consider DVD content vs. HD content. If you own an HD television set, you can still watch DVDs and have them look great… but you could also watch that same content in HD and have it look even better. Your DVD content is effectively being zoomed on an HD set. HD content will show you more detail because it displays more distinct pixels of information in the same amount of physical space.
The same thing should end up being true of Web sites when they are magnified on screens, and that’s what this proposal is trying to make clear. Note that I’m not even talking about particularly high DPI screens. Many people who own 1920×1200 laptop screens already wish they could easily scale their whole UI now. (This is something that computer geeks with keen vision sometimes can’t appreciate.
The fact that Web content will be zoomed (either via a browser-level feature or an OS-level feature) is inevitable. It’s important that browsers have agreed-upon behavior for how Web sites should behave when the content is magnified so that designers have the ability to present a higher level of detail to users who would like to have the “HD” Web viewing experience.