[css-color-4] add normref for HSL, see #6910

Author: svgeesus

css-color-4/Overview.bs

@@ -45,6 +45,13 @@ WPT Display: open
 			"title": "A perceptual color space for image processing",
 			"authors": "Björn Ottosson",
 			"date": "December 2020"
+		},
+		"HSL": {
+			"title": "Color spaces for computer graphics",
+			"authors": "George H. Joblove, Donald Greenberg",
+			"journal": "ACM SIGGRAPH Computer Graphics Volume 12 Issue 3 pp 20–25",
+			"date": "August 1978",
+			"href": "https://doi.org/10.1145/965139.807362"
 		}
 	}
 </pre>
@@ -2169,7 +2176,7 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	how to alter an RGB color to produce a lighter variant of the same hue.
 
 	There are several other color schemes possible.
-	One such is the HSL color scheme,
+	One such is the HSL [[!HSL]] color scheme,
 	which is much more intuitive to use,
 	but still maps easily back to RGB colors.