[css-color-5] Add RCS XYZ example

Author: svgeesus

css-color-5/Overview.bs

@@ -1447,13 +1447,13 @@ The Oklch chroma has dropped from 0.251 to 0.117.
 </div>
 
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 <h2 id="color-function">
@@ -1603,6 +1603,20 @@ the allowed [=channel keywords=] are:
 		parsing/color-computed-relative-color.html
 	</wpt>
 
+	<div class="example" id="ex-RCS-XYZ">
+		For example, Relative Color Syntax in the CIE XYZ D65 colorspace
+		is used to generate a color which has exactly half the luminance
+		of the base color:
+
+		<pre class="lang-css">
+			--base: <span class="swatch" style="--color: rgb(73.58% 48.92% 10.45%)"></span> color(display-p3 0.7 0.5 0.1);
+			--dark: <span class="swatch" style="--color: rgb(53.617% 35.186% 6.4237%)"></span> color(from var(--base) xyz-d65 calc(x/2) calc(y/2) calc(z/2));
+		</pre>
+
+		The origin color is color(xyz-d65 0.281 0.253 0.044)
+		and so the relative color is color(xyz-d65 0.14 0.126 0.022).
+	</div>
+
 <h3 id="custom-color">
 	Custom Color Spaces
 </h3>