[css-color-5] Added a second form of the light-dark() function, which takes a pair of images rather than a pair of colors w3c#12513

Author: svgeesus

css-color-5/Overview.bs

@@ -124,7 +124,7 @@ Introduction {#intro}
 
 	<pre class='prod'>
 	&lt;color> = <<color-base>> | currentColor | <<system-color>> | 
-				<<contrast-color()>> | <<device-cmyk()>>  | <<light-dark()>>
+				<<contrast-color()>> | <<device-cmyk()>>  | <<light-dark-color>>
 
 	<dfn>&lt;color-base></dfn> = <<hex-color>> | <<color-function>> | <<named-color>> | <<color-mix()>> | transparent
 	<dfn>&lt;color-function></dfn> = <<rgb()>> | <<rgba()>> |
@@ -141,7 +141,7 @@ Introduction {#intro}
 
 	* ''currentColor'' (which depends on the value of the 'color' property)
 	* a <<system-color>> (which depends on the color mode)
-	* <<light-dark()>> (which depends on the color mode)
+	* <<light-dark-color>> (which depends on the color mode)
 	* <<contrast-color()>> (which depends on the color mode)
 	* <<device-cmyk()>> (which has no colorimetric basis)
 
@@ -2766,9 +2766,17 @@ or any other color or monochrome output device which has been characterized.
 
 	There are two forms of this function: one takes a pair of colors
 	while the other takes a pair of images. 
+	Attempting to use one image and one color
+	will result in a parse-time error.
 
-	<pre class='prod'>
+	<!-- <pre class='prod'>
 		<dfn export>light-dark()</dfn> =  light-dark( [<<color>>, <<color>>] | [<<image>>, <<image>>])
+	</pre> -->
+
+	<pre class='prod'>
+		<dfn export>light-dark()</dfn> = <<light-dark-color>> | <<light-dark-image>>
+		<dfn export><<light-dark-color>></dfn> = light-dark(<<color>>, <<color>>)
+		<dfn export><<light-dark-image>></dfn> = light-dark(<<image>>, <<image>>) 
 	</pre>
 
 	For the color form, this function computes to the computed value of the first color,
@@ -2828,6 +2836,7 @@ or any other color or monochrome output device which has been characterized.
 	<wpt>
 		light-dark-basic.html
 		light-dark-currentcolor.html
+		light-dark-image.html
 		light-dark-inheritance.html
 		light-dark-currentcolor-in-color.html
 		/css/css-pseudo/highlight-styling-004.html
@@ -3776,6 +3785,10 @@ This specification adds a way to ensure adequate contrast for text whose backgro
 </h3>
 
 <ul>
+	<li>Added a second form of the light-dark() function, 
+		which takes a pair of images rather than a pair of colors
+		(<a href="https://github.com/w3c/csswg-drafts/issues/12513">Issue 12513</a>)
+	</li>
 	<li>Added a color-mix() example with three colors, now that it is no longer restricted to just two.</li>
 </ul>