[jentulman]

Headers do seem the best place for a non vendor specific standard to be set. I'd love to see something like

viewport-dpi - the dpi viewport-max - the maximum possible pixel dimension and possibly viewport-current - the pixel dimensions at the time of the request

Decent responsive design should deal with the differing viewport sizes, but it might be nice to get a hint before delivering your page what direction to weight that response in.

[Groxx]

What do you do when using a projector, and your DPI is something like 3? (serious question, I don't know how this is traditionally handled)

[kijin]

Something like device-pixel-ratio would be more useful than DPI. Raw DPI is meaningless unless you take distance into account, as is very clear in the case of the projector.