[css-color-5] Clarify calculating contrast, based on D65 luminance

Author: svgeesus

css-color-5/Overview.bs

@@ -598,8 +598,20 @@ Selecting the most contrasting color: the ''color-contrast()'' function {#colorc
 	(typically a background, but not necessarily),
 	secondly, a list of two or more colors,
 	and thirdly, an optional target <a href="https://www.w3.org/TR/WCAG21/#contrast-minimum">luminance contrast</a> [[!WCAG21]].
-	It selects from that list
-	the first color color to meet or exceed the target contrast.
+
+	For each color in the list,
+	the CIE Luminance (Y) is calculated,
+	relative to a [=D65 whitepoint=].
+
+	For each pair of colors,
+	the WCAG 2.1 contrast is calculated:
+	contrast = (Y<sub>l</sub> + 0.05) / (Y<sub>d</sub> + 0.05)
+	where Y<sub>d</sub> is the luminance of the darker color in the pair
+	and Y<sub>l</sub> is the luminance of the lighter color.
+
+
+	It then selects from that list
+	the first color to meet or exceed the target contrast.
 	If no target is specified,
 	it selects the first color with the highest contrast
 	to the single color.
@@ -1002,11 +1014,11 @@ The grammar of the ''oklab()'' function is extended as follows:
 Within a [=relative color=] syntax ''oklab()'' function,
 the allowed [=channel keywords=] are:
 
-* <dfn value for="lab()">l</dfn> is a <<percentage>>
+* <dfn value for="oklab()">l</dfn> is a <<percentage>>
 	that corresponds to the [=origin color's=] OKLab Lightness
-* <dfn value for="lab()">a</dfn> and <dfn value for="lab()">b</dfn> are <<number>>s
+* <dfn value for="oklab()">a</dfn> and <dfn value for="oklab()">b</dfn> are <<number>>s
 	that correspond to the [=origin color's=] OKLab a and b axes
-* <dfn value for="lab()">alpha</dfn> is a <<percentage>> that corresponds to the [=origin color's=] alpha transparency
+* <dfn value for="oklab()">alpha</dfn> is a <<percentage>> that corresponds to the [=origin color's=] alpha transparency
 
 <h3 id="relative-LCH">Relative LCH colors</h3>
 
@@ -1104,14 +1116,14 @@ The grammar of the ''oklch()'' function is extended as follows:
 Within a [=relative color=] syntax ''oklch()'' function,
 the allowed [=channel keywords=] are:
 
-* <dfn value for="lch()">l</dfn> is a <<percentage>>
+* <dfn value for="oklch()">l</dfn> is a <<percentage>>
 	that corresponds to the [=origin color's=] OKLab Lightness
-* <dfn value for="lch()">c</dfn> is a <<number>>
+* <dfn value for="oklch()">c</dfn> is a <<number>>
 	that corresponds to the [=origin color's=] OKLCH chroma
-* <dfn value for="lch()">h</dfn> is an <<angle>>
+* <dfn value for="oklch()">h</dfn> is an <<angle>>
 	that corresponds to the [=origin color's=] OKLCH hue,
 	normalized to a [0deg, 360deg) range.
-* <dfn value for="lch()">alpha</dfn> is a <<percentage>> that corresponds to the [=origin color's=] alpha transparency
+* <dfn value for="oklch()">alpha</dfn> is a <<percentage>> that corresponds to the [=origin color's=] alpha transparency
 
 
 <h3 id="relative-color-function">Relative color-function colors</h3>