[css-color-4] HWB achromatic check, less handwaving

Author: svgeesus

css-color-4/Overview.bs

@@ -1788,11 +1788,16 @@ 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,
-	so their sum is no larger than 100%,
-	and have been converted into numbers in the range [0,1].
+	first normalizes the white and black components,
+	so their sum is no larger than 100%.
 
 	<pre class="lang-javascript">
+	white /= 100;
+	black /= 100;
+	if (white + black >= 1) {
+		var gray = white / (white + black);
+		return [gray, gray, gray];
+	  }
 	function hwbToRgb(hue, white, black) {
 		var rgb = hslToRgb(hue, 1, .5);
 		for(var i = 0; i &lt; 3; i++) {
@@ -1806,17 +1811,13 @@ Converting HWB colors to sRGB colors</h3>
 <h3 id="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 than 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]);
+		return([hsl[0], white*100, black*100]);
 	}
 </pre>