[css-color-4] define "can't be displayed", fix w3c#5046

Author: svgeesus

css-color-4/Overview.bs

@@ -2178,7 +2178,7 @@ Profiled, Device-dependent Colors</h2>
 
 	The color function takes one or more comma-separated arguments,
 	with each argument specifying a color,
-	and later colors acting as "fallback" if an earlier color can't be displayed
+	and later colors acting as "fallback" if an earlier color [=can't be displayed=]
 	(for example, if the colorspace it specifies hasn't been loaded yet).
 
 	Each argument has the following form:
@@ -2208,12 +2208,6 @@ Profiled, Device-dependent Colors</h2>
 				where the additional inks are spot colors or varnishes
 				that most colors on the page won't use.)
 
-				Values less than 0 or 0%, or greater than 1 or 100%,
-				are not invalid; instead, they are
-				<a>gamut-mapped</a> using a relative colorimetric intent
-				which brings the values within the range 0/0% to 1/100%
-				at computed-value time.
-
 				If a <<string>> is provided,
 				this argument represents an <a>invalid color</a>.
 			<dt>If the colorspace defines named colors
@@ -2233,12 +2227,60 @@ Profiled, Device-dependent Colors</h2>
 	using any CSS color syntax
 	can be provided.
 
+
 	The ''color()'' function represents the color
 	specified by the first of its arguments that represent a <dfn export>valid color</dfn>
 	(that is, the first argument that isn't an <dfn export>invalid color</dfn>).
 	If all of its arguments represent <a>invalid colors</a>,
 	''color()'' represents <a>opaque black</a>.
 
+	A color may be a [=valid color=]
+	but still be outside the range of colors
+	that can be produced by an output device
+	(a screen, projector, or printer).
+	It is said to be <dfn export>out of gamut</dfn>
+	for that colorspace.
+
+	An out of gamut color has component values
+	less than 0 or 0%, or greater than 1 or 100%.
+	These are are not invalid; instead, they are
+	<a>gamut-mapped</a> using a relative colorimetric intent
+	which brings the values within the range 0/0% to 1/100%
+	at computed-value time.
+
+	A color which is either an [=invalid color=] or
+	an [=out-of-gamut=] color
+	<dfn export>can't be displayed</dfn>.
+
+	<div class="example">
+		This very intense lime color is in-gamut for rec.2020:
+		<pre>color(rec2020 0.42053 0.979780 0.00579);</pre>
+		in LCH, that color is
+		<pre>lch(86.6146 160.0000, 136.0088);</pre>
+		in display-p3, that color is
+		<pre>color(display-p3 -0.6112 1.0079 -0.2192);</pre>
+		and is out of gamut for display-p3
+		(red and blue are negative, green is greater than 1).
+		If you have a display-p3 screen, that color is:
+		<ul>
+			<li><em>valid</em></li>
+			<li><em>in gamut</em> (for rec.2020)</li>
+			<li><em>out of gamut</em> (for your display)</li>
+			<li>and so <em>can't be displayed</em></li>
+		</ul>
+		The color used for display will be a less intense color
+		produced automatically by gamut mapping.
+	</div>
+
+	<div class="example">
+		The same color as the previous example is now specified with a fallback
+		<pre>color(rec2020 0.42053 0.979780 0.00579, color(display-p3(0 1 0)));</pre>
+		On a display-p3 screen,
+		because the first-choice color can't be displayed
+		and because a fallback has been given which is in-gamut for the display,
+		the fallback color will be used.
+	</div>
+
 <h3 id="predefined">
 Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'', ''rec2020'' and ''lab'''.</h3>