[css-color-4][editorial] Add note to hwb about Abney effect

Author: svgeesus

css-color-4/Overview.bs

@@ -54,6 +54,10 @@ spec:css-color-5; type:value; for:oklch();
 
 <pre class=biblio>
 	{
+		"Abney": {
+			"title": "Abney effect",
+			"href": "https://en.wikipedia.org/wiki/Abney_effect"
+		},
 		"TIFF": {
 			"href": "https://www.loc.gov/preservation/digital/formats/fdd/fdd000022.shtml",
 			"title": "TIFF Revision 6.0",
@@ -2829,6 +2833,10 @@ HWB Colors: ''hwb()'' function</h2>
 		<span class="swatch" style="--color:hwb(150 20% 10%)"></span>&nbsp;rgb(20% 90% 55%).
 	</div>
 
+	Note: ''hwb()'' does not account for the  Abney effect [[Abney]]
+	which is the perceived hue shift that occurs
+	when white light is added to a monochromatic light source.
+
 	<!-- unbounded hwb
 	Values outside of these ranges
 	are not invalid,