[css-color-4] export polar and rectangular terms

Author: svgeesus

css-color-4/Overview.bs

@@ -293,12 +293,17 @@ Representing Colors: the <<color>> type</h2>
 		<<color()>>.
 
 	Of those, <<hsl()>>, <<hsla()>>, <<hwb()>> and <<lch()>>
-		are cylindrical polar representations,
+		are <dfn export>cylindrical polar color</dfn> representations,
 		which specify color using a <<hue>> angle,
 		a central axis representing lightness
 		(black-to-white),
 		and a radius representing saturation or chroma
-		(how far the color is from a neutral grey)
+		(how far the color is from a neutral grey).
+	The other colorspaces are
+		<dfn export>rectangular orthogonal color</dfn> representations,
+		which specify color using three
+		(or more, in the case of CMYK or CMYKOGV)
+		orthogonal component axes.
 
 	<!--  <<device-cmyk()>> does not actually specify a color; it can't be converted to Lab -->
 

css-color-5/Overview.bs

@@ -760,25 +760,37 @@ occurs by linearly interpolating each component of the computed value of the col
 separately, in that colorspace.
 This provides Web compatibility; legacy sRGB content interpolates in the sRGB space by default.
 
-Issue: Should gamut mapping occur before or after interpolation?
-
-Issue(4928): How to handle achromatic interpolation?
 
 Interpolating to or from ''<color>/currentcolor'' is possible.
 The numerical value used for this purpose is the used value.
 
 Color space for interpolation {#interpolation-space}
 ------------------------------
 
-If colors are not in the same color space, they are first converted to Lab and and interpolated as Lab colors.
+Issue: Should gamut mapping occur before or after interpolation?
+
+If colors are not in the same color space,
+they 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.
 
 Issue: Should the colors at 0% and 100% be serialized with their original color spaces or converted to the interpolation space?
 
+Interpolating with alpha {#interpolation-alpha}
+------------------------
+
+When the colors to be interpolated are not fully opaque,
+they are transformed into <dfn export>premultiplied color values</dfn>
+as follows:
+
+ * For rectangular coordinate systems
+
 Hue interpolation {#hue-interpolation}
 -------------------
 
+Issue(4928): How to handle achromatic interpolation?
+
 For color functions with a hue angle (LCH, HSL, HWB etc), there are multiple ways to interpolate.
 We typically want to avoid arcs over 360 for the difference between the angles, as they are rarely desirable,
 so in most cases angles are fixed up prior to interpolation so that per-component interpolation is done over less than 360 degrees, often less than 180.