device-pixel-ratio as obtained by
The above box has been specified to have a width of 2.083em which should amount to 200px (The font-size is 6em).
This image is 200px wide and 58px high.
All other font-sizes are browser defaults (
font-size is set to 100%). This page uses:
<meta name="viewport" content="width=device-width, minimum-scale=1.0">
See also PPK's test.