<resolution>

Summary

The <resolution> CSS data types, used in media queries, denotes the density of pixels of an output device, its resolution. It is a <number> immediately followed by a unit of resolution (dpi, dpcm, ...). Like for any CSS dimension, there is no space between the unit literal and the number.

On screens, the length is related to CSS inches, centimeters or pixels, not on physical values.

Even if all units represent the same resolution for the value 0, the unit may not be omitted in that case as it isn't a <length>: 0 is invalid and does not represent 0dpi, 0dpcm, nor 0dppx.

Units

dpi: This unit represents the number of dots per inch. A screen typically contains 72 or 96 dpi; a printed document usually reach much greater dpi. As 1 inch is 2.54 cm, 1dpi ≈ 0.39dpcm.
dpcm: This unit represents the number of dots per centimeter. As 1 inch is 2.54 cm, 1dpcm ≈ 2.54dpi.
dppx: This unit represents the number of dots per px unit. Due to the 1:96 fixed ratio of CSS in to CSS px, 1dppx is equivalent to 96dpi, that corresponds to the default resolution of images displayed in CSS as defined by image-resolution.

Examples

Here are some correct uses of <resolution> values:

96dpi                                              Correct use: a <number> (here an <integer>) followed by the unit.
@media print and (min-resolution: 300dpi) { ... }  Correct use in the context of a media query.

Here are some incorrect uses:

72 dpi                                             Incorrect: no spaces allowed between the <number> and the unit.
ten dpi                                            Incorrect: only digits must be used.
0                                                  Incorrect: the unit can be omitted for 0 values only for <length>.

Specifications

Specification	Status	Comment
CSS Values and Units Module Level 3 The definition of '<resolution>' in that specification.	Candidate Recommendation	Factorization of the type in a more generic specification. No change
CSS Image Values and Replaced Content Module Level 3 The definition of '<resolution>' in that specification.	Candidate Recommendation	Added the `dppx` unit
Media Queries The definition of '<resolution>' in that specification.	Recommendation	Initial definition

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari (WebKit)
Basic support	29	3.5 (1.9.1)^[1]	9	9.5	No support^[2]
`dppx`	29	16.0 (16.0)	?	12.10	?

Feature	Android	Firefox Mobile (Gecko)	IE Phone	Opera Mobile	Safari Mobile
Basic support	No support^[2]	(Yes)	?	(Yes)	No support^[2]
`dppx`	?	16.0 (16.0)	?	12.10	?

[1] Before Firefox 8 (Gecko 8.0), it erroneously accepted only CSS dimensions that were <integer> followed by the unit. From that version, it supports any valid CSS dimensions (<number> immediately followed by the unit).

[2] Webkit engine does not support CSS resolution query as in the specification, the use of the non-standard device-pixel-ratio query is needed for browsers Safari, see bug 16832.