[css-color-hdr] Added color function grammar, extending CSS Color 5

Author: svgeesus

css-color-hdr/Overview.bs

@@ -243,12 +243,42 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
 
 	Percentages are normalized in the same way as ''color-mix()''.
 
+<h2 id="color-function">
+	Specifying Predefined and Custom Color Spaces: the ''color()'' Function
+</h2>
+
+	The ''color()'' function allows a color to be specified
+	in a particular, given [=color space=]
+	(rather than the implicit sRGB color space that most of the other color functions operate in).
+
+	In this specification the ''color()'' function is extended
+	to allow <a href="#predefined-HDR">predefined color spaces for HDR</a>,
+	in addition to the predefined SDR spaces from [[css-color-4#predefined]]
+	and the relative color syntax from [[css-color-5#relative-colors]].
+
+	Its syntax is now as follows:
+
+	<pre class='prod'>
+		<dfn>color()</dfn> = color( [from <<color>>]? <<colorspace-params>> [ / [ <<alpha-value>> | none ] ]? )
+		<dfn>&lt;colorspace-params></dfn> = [<<custom-params>> | <<predefined-rgb-params>> | 
+			<<predefined-polar-params>> | <<predefined-rectangular-params>> | <<xyz-params>>]
+		<dfn>&lt;custom-params></dfn> = <<dashed-ident>> [ <<number>> | <<percentage>> | none ]+
+		<dfn>&lt;predefined-rgb-params></dfn> = <<predefined-rgb>> [ <<number>> | <<percentage>> | none ]{3}
+		<dfn>&lt;predefined-polar-params></dfn> = jzczhz [ <<number>> | <<percentage>> | none ]{2} [ <<hue>> | none]
+		<dfn>&lt;predefined-rectangular-params></dfn> = <<predefined-rectangular>> [ <<number>> | <<percentage>> | none ]{3}
+		<dfn>&lt;predefined-rgb></dfn> = srgb | srgb-linear | display-p3 | a98-rgb | prophoto-rgb | rec2020 | 
+			rec2100-pq | rec2100-hlg | rec2100-linear
+		<dfn>&lt;predefined-rectangular></dfn> = jzazbz | ictcp
+		<dfn>&lt;xyz-params></dfn> = <<xyz>> [ <<number>> | <<percentage>> | none ]{3}
+		<dfn>&lt;xyz></dfn> = xyz | xyz-d50 | xyz-d65
+	</pre>
+
 Predefined color spaces for HDR: {#predefined-HDR}
 =====================
 
 	In addition to the <a href="https://drafts.csswg.org/css-color-4/#predefined">SDR
 	color spaces</a> defined by CSS Color 4, the following HDR color spaces are defined
-	for use in the color function [[css-color-4#color-function]].
+	for use in the color function.
 
 	Serialization of values in these new color spaces is identical to the description in [[css-color-4#serializing-color-values]].