[mediaqueries] Clarify that the values work for larger gamuts too. Add several clarifying notes and a few issues.

Author: tabatkins

mediaqueries/Overview.bs

@@ -1458,9 +1458,9 @@ color-gamut</h3>
 	</pre>
 
 	The 'color-gamut' media feature describes the approximate range of colors
-	that are supported by the browser and display device.
-	That is, if the browser receives content with colors in the specified space
-	it can cause the display to render the appropriate color,
+	that are supported by the UA and output device.
+	That is, if the UA receives content with colors in the specified space
+	it can cause the output device to render the appropriate color,
 	or something appropriately close enough.
 
 	Note: The query uses approximate ranges for a few reasons.
@@ -1476,25 +1476,31 @@ color-gamut</h3>
 	tagged with color profiles,
 	to the user.
 
-	Values are defined as:
-
 	<dl dfn-type=value dfn-for="@media/color-gamut">
 		<dt><dfn>srgb</dfn>
 		<dd>
-			The value ''srgb'' represents a display that corresponds to the sRGB Color Space [[!SRGB]].
+			The output device can support approximately the sRGB gamut [[!SRGB]] or more.
 
-			It is expected that the vast majority of color displays
+			Note: It is expected that the vast majority of color displays
 			will be able to return true to a query of this type.
 
 		<dt><dfn>p3</dfn>
 		<dd>
-			The value ''p3'' represents a display that can handle colors in approximately the gamut
-			specified by the DCI P3 Color Space.
+			The output device can support approximately the gamut
+			specified by the DCIP3 Color Space or more.
+
+			Note: The ''p3'' gamut is larger than and includes the ''srgb'' gamut.
+
+			Issue: Add ref for DCIP3.
 
 		<dt><dfn>rec2020</dfn>
 		<dd>
-			The value ''rec2020'' represents a display that can handle colors in approximately the gamut
-			specified by the ITU-R Recommendation BT.2020 Color Space.
+			The output device can support approximately the gamut
+			specified by the ITU-R Recommendation BT.2020 Color Space or more.
+
+			Note: The ''rec2020'' gamut is larger than and includes the ''p3'' gamut.
+
+			Issue: Add ref for BT.2020.
 	</dl>
 
 	<div class="example">
@@ -1506,6 +1512,26 @@ color-gamut</h3>
 		</pre>
 	</div>
 
+	Note: An output device can return true for multiple values of this media feature,
+	if its full output gamut is large enough,
+	or one gamut is a subset of another supported gamut.
+	As a result,
+	this feature is best used in an "ascending" fashion--
+	set a base value when ''(color-gamut: srgb)'' is true,
+	then override it if ''(color-gamut: p3)'' is true, etc.
+
+	Note: Some output devices,
+	such as monochrome displays,
+	cannot support even the ''srgb'' gamut.
+	To test for these devices,
+	you can use this feature in a negated boolean-context fashion:
+	''not (color-gamut)''.
+
+	Issue: Or should we add a value specifically for the "less than sRGB" case,
+	like <css>narrow</css>?
+	It would have to work differently than the others,
+	and only match if the gamut was extra-low.
+
 
 
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