Retina display support

I'm not really sure what you mean by Retina-specific hacks. As a notebook Retina-display user, I can say in 6 months of daily browsing, I have yet to find a website that doesn't render correctly.

The browser still lays things out identically, it just renders extra pixels for text+vector graphics, and anti-aliases raster images. That's the whole reason why it's implemented using a pixel-ratio. Everything automatically works.

Coding webpages in real units is impractical anyways, as you don't know the device it'll be displayed on. No one on a phone wants a webpage to be initially zoomed to be 10 inches wide when their screen is only 2-3 inches wide. The first thing they'll do is zoom out to figure out where to scroll to before zooming back in. Same idea for a tablet. Laptops/desktops are a little easier, but you still don't know the size of the display (9" netbook or 30" desktop?), how far away the user is sitting (HTPC or netbook?), and how good their eyesight is. Ultimately, it's up to the device-maker and end user.

All the web-developer needs is a reference unit. The "pixel", while poorly named, already provides this. Each device maker can decide how to handle the scaling ratio for a given form-factor. Ideally one day, once desktop OS's are truly resolution independent, they'll let end-users pick their "ideal" ratio with a slider somewhere that affects the entire UI.

Yes you'll need to start serving up higher resolution images soon or later, but this is caused by hi-dpi displays in general and not just Retina(TM) displays. It's already possible to query the device pixel ratio in a browser-independent way, or alternately blindly serve up high-res images to everyone.