Editorial rewrite of HDR text (w3c#5041)

Author: frivoal

mediaqueries-4/Overview.bs

@@ -1706,26 +1706,48 @@ color depth, and contrast ratio that are supported by the UA and output device.
 <dl dfn-type=value dfn-for="@media/dynamic-range">
 	<dt><dfn>high</dfn>
 	<dd>
-		If the following are true for the UA and the output device:
-		* max brightness is considered high
-		* color depth is greater than 24 bit
-		* has a <a>high contrast ratio</a>
+		The combination of the User Agent and the output device
+		fulfill all of the following criteria:
+		* it has a [=high peak brightness=]
+		* it has a [=high contrast ratio=]
+		* its color depth is greater than 24 bit or 8 bit per color component of RGB
 	<dt><dfn>standard</dfn>
 	<dd>
-		If the following are true for the UA and the output device:
-		* max brightness is considered standard
-		* color depth is 24 bit or 8 bit per color component of RGB
-		* has a <a>low contrast ratio</a>
+		One or more of the criteria for a ''high'' 'dynamic-range'
+		is not fulfilled.
 </dl>
 
 <h3 id="contrast-brightness-of-display">
 	Determining contrast and brightness of display</h3>
 
-This text is non-normative. There currently is no agreed upon standardization of
-<dfn export>high contrast ratio</dfn>, <dfn export>low contrast ratio</dfn>,
-max brightness or low brightness as it relates to displays. As such, the
-determination for '@media/dynamic-range' and '@media/video-dynamic-range' will
-be defined by the user agent.
+	<dfn export>Peak brightness</dfn> refers to how bright the brightest point
+	a light-emitting device such as an LCD screen can produce,
+	or in the case of a light reflective device such as paper or e-ink,
+	the point at which it least absorbs light.
+
+	Note: Some devices can only produce their [=peak brightness=]
+	for brief periods of time or on a small portion of their surface at any given time.
+
+	The <dfn export>contrast ratio</dfn> is the ratio of the luminance
+	of the brightest color to that of the darkest color
+	that the system is capable of producing.
+
+	This specification does not define precise ways
+	by which these qualities can be measured;
+	it also lets the User Agent determine
+	what counts as a <dfn for="contrast ratio" lt="high contrast ratio">high</dfn> [=contrast ratio=]
+	and as a <dfn for="peak brightness" lt="high peak brightness">high</dfn> [=peak brightness=].
+	User Agents must nonetheless attempt to conform to the following intent:
+	a device capable of [=high peak brightness=]
+	can display “brighter than white” highlights,
+	and a simultaneous ability to do so
+	while also presenting deep blacks
+	(rather than an overall bright but washed out image)
+	is indicative of a [=high contrast ratio=].
+
+	Note: The determination for '@media/dynamic-range' and '@media/video-dynamic-range'
+	will be vary depending on the User Agent,
+	but is expected to have broadly dependable semantics.
 
 <h2 id="video-prefixed-features">Video Prefixed Features</h2>