[css-color-4] Don't force non-legacy colors to interpolate in a gamma-encoded space w3c#5883

Author: svgeesus

css-color-4/Overview.bs

@@ -4364,11 +4364,32 @@ Interpolation</h2>
 	animations, and
 	color mixing and color modification functions.
 
-	In general, interpolation between <<color>> values of the same [=colorspace=]
+	In general, interpolation between <<color>> values
 	occurs by linearly interpolating each component of the computed value of the color
-	separately, in that colorspace.
-	This provides Web compatibility; legacy sRGB content interpolates in the sRGB space by default.
+	separately, in Lab.
+
+<!--
+
+	Historically the existing specifications used interpolation in gamma-encoded sRGB.
+	This gave poor results, and was sRGB-only.
+	This specification improves the situation by
+
+	- using a perceptually uniform interpolation space by default
+	- enabling interpolation between any color
+	  (which has a colorimetric interpretation, which is all of them except for device-cmyk and even then, only in corner cases)
+	- allowing individual specifications to override this
+	  (for example compositing should be in linear-light XYZ)
+	  (for example, clor-mix defaults to LCH but allows other spaced to be explicitly specified)
+	- maintaining Web compat with special-case where all the colors to be interpolated use legacy sRGB
+	- providing an opt-in for sRGB colors
+
+	The legacy specifications will over time be updated to add support for non-sRGB colors
+
+	This is all in a comment because we don't make promises about future specifications
 
+	But rest assured, they are all being targeted.
+
+-->
 
 	Interpolating to or from ''<color>/currentcolor'' is possible.
 	The numerical value used for this purpose is the used value.
@@ -4386,12 +4407,23 @@ Color space for interpolation</h3>
 
 	Issue: Should gamut mapping occur before or after interpolation?
 
-	If colors are not in the same color space,
-	they are first converted to Lab
+	Colors are first converted to Lab
 	and then interpolated as Lab colors.
 	Host syntax can override the interpolation color space and specify which color space is used for interpolation.
 	For example, 'color-mix' and 'color-adjust' override the default to LCH.
 
+	The exception is that if all the <<color>>s to be interpolated
+	are sRGB values, and they are all expressed in a legacy syntax
+	such as hex colors, named colors,
+	''rgb()'', ''hsl()'' or ''hwb()''
+	(and the equivalent alpha-including forms)
+	then they are interpolated in gamma-encoded sRGB space.
+
+	This provides Web compatibility; legacy sRGB content interpolates in the sRGB space by default.
+
+	This also means that authors can choose to opt-in to Lab interpolation
+	by using the non-legacy ''color(srgb r g b)'' form.
+
 <h3 id="interpolation-alpha">
 Interpolating with alpha</h3>