[css-color-4] clarify RGB-to-Lab & Lab-to-RGB, fix w3c#4647

Author: svgeesus

css-color-4/Overview.bs

@@ -1268,10 +1268,10 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	across hues.
 
 	<div class="example">
-		For example, <span class="swatch" style="--color: blue"></span>&nbsp;''blue'''
+		For example, <span class="swatch" style="--color: blue"></span>&nbsp;''blue''
 		is represented in HSL as
-		<span class="swatch" style="--color: blue"></span>&nbsp;''hsl(240deg 100% 50%)
-		while <span class="swatch" style="--color: yellow"></span>&nbsp;''yellow'''
+		<span class="swatch" style="--color: blue"></span>&nbsp;''hsl(240deg 100% 50%)''
+		while <span class="swatch" style="--color: yellow"></span>&nbsp;''yellow''
 		is <span class="swatch" style="--color: yellow"></span>&nbsp;''hsl(60deg 100% 50%)''.
 		Both have an HSL Lightness of 50%,
 		but clearly the yellow looks much lighter than the blue.
@@ -1302,24 +1302,24 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 		<span class="swatch" style="--color: hsl(220 100% 50%)"></span>&nbsp;''hsl(220deg 100% 50%)''
 		and
 		<span class="swatch" style="--color: hsl(250 100% 50%)"></span>&nbsp;''hsl(250deg 100% 50%)''
-		have an HSL hue difference of 30deg and look fairly similar,
+		have an HSL hue difference of 250-220 = <strong>30</strong>deg and look fairly similar,
 		while another pair of colors
 		<span class="swatch" style="--color: hsl(50 100% 50%)"></span>&nbsp;''hsl(50deg 100% 50%)''
 		and
 		<span class="swatch" style="--color: hsl(80 100% 50%)"></span>&nbsp;''hsl(80deg 100% 50%)'',
-		which also have a hue difference of 30deg, look very different.
+		which also have a hue difference of 80-50 = <strong>30</strong>deg, look very different.
 
 		In LCH, the same pair of colors
 		<span class="swatch" style="--color: hsl(220 100% 50%)"></span>&nbsp;''lch(42.1% 97.4 290.6)''
 		and
 		<span class="swatch" style="--color: hsl(250 100% 50%)"></span>&nbsp;''lch(30.8% 129.7 302.1)''
-		have a hue difference of 11.5deg
+		have a hue difference of 302.1-290.6 = <strong>11.5</strong>deg
 		while the second pair
 		<span class="swatch" style="--color: hsl(50 100% 50%)"></span>&nbsp;''lch(86.8% 86.2 87.3)''
 		and
 		<span class="swatch" style="--color: hsl(80 100% 50%)"></span>&nbsp;''lch(92.0% 98.8 119.1)''
-		have a hue difference of 31.8deg
-		reflecting the visual separation of hues.
+		have a hue difference of 119.1-87.3 = <strong>31.8</strong>deg,
+		correctly reflecting the visual separation of hues.
 
 	</div>
 
@@ -2253,10 +2253,10 @@ Specifying Lab and LCH: the ''lab()'' and ''lch()'' functional notations</h3>
 	</div>
 
 <h3 id='rgb-to-lab'>
-Converting sRGB colors to Lab colors</h3>
+Converting RGB colors to Lab colors</h3>
 
-	Conversion from sRGB to Lab requires several steps,
-	although in practice all but the first step are linear calculations and can be combined.
+	Conversion from an RGB colorspace to Lab requires several steps,
+	although in practice all but the first step are linear calculations and can be combined. Taking sRGB as an example:
 
 	<!-- make these steps links to the appropriate section in a calculations appendix -->
 	<ol>
@@ -2271,20 +2271,25 @@ Converting sRGB colors to Lab colors</h3>
 		<li>Convert D50-adapted XYZ to Lab
 	</ol>
 
-	There is sample JavaScript code for this conversion in [[#color-conversion-code]].
+	There is sample JavaScript code for this conversion
+	for various RGB colorspaces, in [[#color-conversion-code]].
 
 <h3 id='lab-to-rgb'>
-Converting Lab colors to sRGB  colors</h3>
+Converting Lab colors to RGB colors</h3>
 
-	Conversion from Lab to sRGB also requires multiple steps,
-	and again in practice all but the last step are linear calculations and can be combined.
+	For display on a screen,
+	Lab colors will need to be converted
+	to the colorspace of the display.
+
+	Conversion from Lab to an RGB space also requires multiple steps,
+	and again in practice all but the last step are linear calculations and can be combined. Taking display-p3 as an example,
 
 	<ol>
 		<li>Convert Lab to (D50-adapted) XYZ
 		<li>Convert from a D50 whitepoint (used by Lab) to the D65 whitepoint used
-			in sRGB, with the Bradford transform
-		<li>Convert from (D65-adapted) CIE XYZ to linear sRGB
-		<li>Convert from linear-light sRGB to sRGB (do gamma encoding)
+			in display-p3, with the Bradford transform
+		<li>Convert from (D65-adapted) CIE XYZ to linear-light display-p3
+		<li>Convert from linear-light display-p3 to display-p3 (do gamma encoding)
 	</ol>
 
 <h3 id='lab-to-lch'>