[css-color-5] Clarify that relative color keywords can only be used if an origin color is specified

Author: LeaVerou

css-color-5/Overview.bs

@@ -240,7 +240,7 @@ Adjusting colors: the ''color-adjust'' function {#coloradjust}
 Relative color syntax {#relative-colors}
 --------------------------------------------------------
 
-Besides specifying absolute coordinates, all color functions can also be used with a *relative syntax* to produce colors in the function's target color space, based on an existing color. This syntax consists of the keyword `from`, a <<color>> value, and optionally numerical coordinates specific to the color function. To allow calculations on the original color's coordinates, there are single-letter keywords for each coordinate and `alpha` that corresponds to the color's alpha. If no coordinates are specified, the function merely converts the origin color to the target function's color space.
+Besides specifying absolute coordinates, all color functions can also be used with a *relative syntax* to produce colors in the function's target color space, based on an existing color (henceforth referred to as "origin color"). This syntax consists of the keyword `from`, a <<color>> value, and optionally numerical coordinates specific to the color function. To allow calculations on the original color's coordinates, there are single-letter keywords for each coordinate and `alpha` that corresponds to the color's alpha. If no coordinates are specified, the function merely converts the origin color to the target function's color space.
 
 The following sections outline the relative color syntax for each color function.
 
@@ -256,7 +256,7 @@ The grammar of the ''rgb()'' function is extended as follows:
 <dfn>&lt;alpha-value></dfn> = <<number>> | <<percentage>>
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'r' is a <<percentage>> that corresponds to the origin color's red channel after its conversion to sRGB
 - 'g' is a <<percentage>> that corresponds to the origin color's green channel after its conversion to sRGB
@@ -272,7 +272,7 @@ The grammar of the ''hsl()'' function is extended as follows:
 <dfn>&lt;hue></dfn> = <<number>> | <<angle>>
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'h' is a <<number>> that corresponds to the origin color's HSL hue after its conversion to sRGB, normalized to a [0, 360) range.
 - 's' is a <<percentage>> that corresponds to the origin color's HSL saturation after its conversion to sRGB
@@ -287,7 +287,7 @@ The grammar of the ''hwb()'' function is extended as follows:
 	<dfn>hwb()</dfn> = hwb([from <<color>>]? <<hue>> <<percentage>> <<percentage>> [ / <<alpha-value>> ]? )
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'h' is a <<number>> that corresponds to the origin color's HWB hue after its conversion to sRGB
 - 'w' is a <<percentage>> that corresponds to the origin color's HWB whiteness after its conversion to sRGB
@@ -302,7 +302,7 @@ The grammar of the ''lab()'' function is extended as follows:
 <dfn>lab()</dfn> = lab([from <<color>>]? <<percentage>> <<number>> <<number>> [ / <<alpha-value>> ]? )
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'l' is a <<percentage>> that corresponds to the origin color's CIE Lightness
 - 'a' is a <<number>> that corresponds to the origin color's CIELab a axis
@@ -317,7 +317,7 @@ The grammar of the ''lch()'' function is extended as follows:
 <dfn>lch()</dfn> = lch([from <<color>>]? <<percentage>> <<number>> <<hue>> [ / <<alpha-value>> ]? )
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'l' is a <<percentage>> that corresponds to the origin color's CIE Lightness
 - 'c' is a <<number>> that corresponds to the origin color's LCH chroma
@@ -337,7 +337,7 @@ The grammar of the ''gray()'' function is extended as follows:
 <dfn>gray()</dfn> = gray([from <<color>>]? <<number>>  [ / <<alpha-value>> ]? )
 </pre>
 
-The following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
+When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:
 
 - 'l' is a <<percentage>> that corresponds to the origin color's CIE Lightness
 - 'alpha' is a <<percentage>> that corresponds to the origin color's alpha transparency