As far as I can tell, the best way to do this (for example, when you want to test Retina visuals on a machine that doesn’t have a Retina display, like a MacBook Air), you can use Firefox, go to
about:config, and change
layout.css.devPixelsPerPx from -1 to 2.
You can also do it in Chrome, but it is fiddlier:
- Zoom to 200%
- At the bottom of the JS console (
Option-Command-J), click on "Emulation"
- Check the "Emulate screen" box
- Set "Device pixel ratio" to 2
- Make sure "Emulate viewport" is unchecked (otherwise the content gets resized)
- Uncheck "Shrink to fit" (it doesn’t seem to do anything useful)
- Uncheck "Enable text autosizing"