[css-color-4] add reverse conversion, sRGB to HWB

Author: svgeesus

css-color-4/Overview.bs

@@ -1570,7 +1570,8 @@ Converting HWB colors to sRGB colors</h3>
 
 	Converting an HWB color to sRGB is straightforward,
 	and related to how one converts HSL to RGB.
-	The following Javascript implementation of the algorithm assumes that the white and black components have already been normalized,
+	The following Javascript implementation of the algorithm
+	assumes that the white and black components have already been normalized,
 	so their sum is no larger than 100%,
 	and have been converted into numbers in the range [0,1].
 
@@ -1585,6 +1586,23 @@ Converting HWB colors to sRGB colors</h3>
 	}
 	</pre>
 
+<h3 class="rgb-to-hwb">Converting sRGB colors to HWB colors</h3>
+
+	Conversion in the reverse direction proceeds similarly.
+	The following Javascript implementation
+	leaves white and black
+	in the range [0,1]
+	rather thn converting them to [0.0%,100.0%].
+
+<pre class="lang-javascript">
+	function rgbToHwb(red, green, blue) {
+		var hsl = rgbToHsl(rgb);
+		var white = Math.min(...rgb);
+		var black = 1 - Math.max(...rgb);
+		return([hsl[0], white, black]);
+	}
+</pre>
+
 <h3 id='hwb-examples'>
 Examples of HWB Colors</h3>