[css-color-4][editorial] Add a diagram showing imaginary colors in CIE Lab

Author: svgeesus

css-color-4/Overview.bs

@@ -5403,6 +5403,26 @@ Note: This section provides important context for the specific requirements desc
 	Those colors tend to come from calculations
 	such as "make this color 100x as saturated".
 
+	<div class="example" id="ex-lab-corners">
+		For example, while theoretically unbounded,
+		the CIE Lab <em>(a,b)</em> plane is often implemented
+		such that <em>a</em> and <em>b</em> are constrained
+		to the range ±127.
+
+		Three of those four corners are outside the spectral locus
+		and thus correspond to imaginary colors.
+
+		<figure id="fig-lab-corners">
+			<object data="./images/UCS-rec2020-labcorners.svg"></object>
+			<figcaption>The four corners of the ±127 (a,b) plane, 
+				on a UCS chromaticity diagram. 
+				The outer shape is the spectral locus.
+				The larger triangle is the rec2020 gamut,
+				while the smaller is sRGB.
+			</figcaption>
+		</figure>
+	</div>
+
 <h4 id="GM-clip">
 Clipping</h4>
 
@@ -5463,7 +5483,7 @@ Clipping</h4>
 		This is a good result; 
 		the hue angle and lightness have barely changed
 		but the chroma is somewhat reduced, as expected.
-     
+
 		In terms of percentages of linear-light red green and blue,
 		the red and green are identical 
 		while the blue is -1.46% higher.
@@ -5484,7 +5504,7 @@ Clipping</h4>
 		<code>color(display-p3 0 1 0)</code>
 		which in linear-light is, again, 
 		<code>color(display-p3-linear 0 1 0)</code>
-    	(component values of exactly 0 or 1 are unaffected by gamma encoding)
+		(component values of exactly 0 or 1 are unaffected by gamma encoding)
 		which, for comparison, is 
 		<code>oklch(84.88% 0.3685 145.6)</code>.