[css-values] Define 'device pixel'. w3c#7287

Author: tabatkins

css-values-3/Overview.bs

@@ -1318,12 +1318,12 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	and devices with unusual viewing distances,
 	it is recommended instead that the [=anchor unit=] be the [=pixel unit=].
 	For such devices it is recommended that the [=pixel unit=]
-	refer to the whole number of device pixels that best approximates the reference pixel.
+	refer to the whole number of [=device pixels=] that best approximates the reference pixel.
 
 	Note: If the anchor unit is the pixel unit,
 	the physical units might not match their physical measurements.
 	Alternatively if the anchor unit is a physical unit,
-	the pixel unit might not map to a whole number of device pixels.
+	the pixel unit might not map to a whole number of [=device pixels=].
 
 	Note: This definition of the [=pixel unit=] and the [=physical units=]
 	differs from the earlier editions of CSS1 and CSS2.
@@ -1347,7 +1347,7 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	css/CSS2/values/units-009.xht
 	</wpt>
 
-	The <dfn export>reference pixel</dfn> is the visual angle of one pixel on a device with a pixel density of 96dpi
+	The <dfn export>reference pixel</dfn> is the visual angle of one pixel on a device with a [=device pixel=] density of 96dpi
 	and a distance from the reader of an arm's length.
 	For a nominal arm's length of 28 inches,
 	the visual angle is therefore about 0.0213 degrees.
@@ -1400,6 +1400,15 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	css/css-values/absolute-length-units-001.html
 	</wpt>
 
+	A <dfn export>device pixel</dfn> is the smallest unit of area on the device output
+	capable of displaying its full range of colors.
+	For typical color screens, it's a square or somewhat rectangular region
+	containing a red, green, and blue subpixel.
+	Many non-traditional outputs exist that can blur this definition,
+	such as by displaying some colors at higher resolutions.
+	Such devices still expose some equivalent notion of "device pixel",
+	however.
+
 <h2 id="other-units">
 Other Quantities</h2>
 

css-values-4/Overview.bs

@@ -2272,12 +2272,12 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	and devices with unusual viewing distances,
 	it is recommended instead that the [=anchor unit=] be the [=pixel unit=].
 	For such devices it is recommended that the [=pixel unit=]
-	refer to the whole number of device pixels that best approximates the reference pixel.
+	refer to the whole number of [=device pixels=] that best approximates the reference pixel.
 
 	Note: If the [=anchor unit=] is the [=pixel unit=],
 	the [=physical units=] might not match their physical measurements.
 	Alternatively if the [=anchor unit=] is a [=physical unit=],
-	the [=pixel unit=] might not map to a whole number of device pixels.
+	the [=pixel unit=] might not map to a whole number of [=device pixels=].
 
 	Note: This definition of the [=pixel unit=] and the [=physical units=]
 	differs from the earlier editions of CSS1 and CSS2.
@@ -2301,7 +2301,7 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	css/CSS2/values/units-009.xht
 	</wpt>
 
-	The <dfn export>reference pixel</dfn> is the visual angle of one pixel on a device with a pixel density of 96dpi
+	The <dfn export>reference pixel</dfn> is the visual angle of one pixel on a device with a [=device pixel=] density of 96dpi
 	and a distance from the reader of an arm's length.
 	For a nominal arm's length of 28 inches,
 	the visual angle is therefore about 0.0213 degrees.
@@ -2354,6 +2354,15 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 	css/css-values/absolute-length-units-001.html
 	</wpt>
 
+	A <dfn export>device pixel</dfn> is the smallest unit of area on the device output
+	capable of displaying its full range of colors.
+	For typical color screens, it's a square or somewhat rectangular region
+	containing a red, green, and blue subpixel.
+	Many non-traditional outputs exist that can blur this definition,
+	such as by displaying some colors at higher resolutions.
+	Such devices still expose some equivalent notion of "device pixel",
+	however.
+
 <h2 id="other-units">
 Other Quantities</h2>