[css-color] Make the alpha optional in the *a() color functions too, per resolution. Simplify grammar. Add note about aliasing.

Author: tabatkins

css-color/Overview.bs

@@ -171,8 +171,8 @@ The RGB functions: ''rgb()'' and ''rgba()''</h3>
 	Its syntax is:
 
 	<pre class='prod'>
-	<dfn>rgb()</dfn> = rgb( <<rgb-component>>#{3} [, <<alpha-value>> ]? )
-	<dfn>rgba()</dfn> = rgba( <<rgb-component>>#{3} , <<alpha-value>> )
+	<dfn>rgb()</dfn> = rgb( <<rgb-component>>#{3} , <<alpha-value>>? )
+	<dfn>rgba()</dfn> = rgba( <<rgb-component>>#{3} , <<alpha-value>>? )
 	<dfn>&lt;rgb-component></dfn> = <<number>> | <<percentage>>
 	<dfn>&lt;alpha-value></dfn> = <<number>> | <<percentage>>
 	</pre>
@@ -197,7 +197,6 @@ The RGB functions: ''rgb()'' and ''rgba()''</h3>
 
 	Issue: The transfer function is undefined outside the range 0% to 100%.
 
-
 	The final argument, the <<alpha-value>>, specifies the alpha of the color.
 	If given as a <<number>>, the useful range of the value is ''0''
 	(representing a fully transparent color)
@@ -209,6 +208,10 @@ The RGB functions: ''rgb()'' and ''rgba()''</h3>
 	but are clamped to the ranges defined here at computed-value time.
 	If omitted, it defaults to ''100%''.
 
+	Note: For legacy reasons, the ''rgb()'' function and ''rgba()'' functions both exist,
+	but are aliases of each other.
+	Both of them allow specifying an RGB color with an optional alpha value.
+
 <h3 id='hex-notation'>
 The RGB hexadecimal notations: ''#RRGGBB''</h3>
 
@@ -709,8 +712,8 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	The syntax of the ''hsl()'' function is:
 
 	<pre class='prod'>
-	<dfn>hsl()</dfn> = hsl( <<hue>>, <<percentage>>, <<percentage>> [, <<alpha-value>> ]? )
-	<dfn>hsla()</dfn> = hsla( <<hue>>, <<percentage>>, <<percentage>>, <<alpha-value>> )
+	<dfn>hsl()</dfn> = hsl( <<hue>>, <<percentage>>, <<percentage>>, <<alpha-value>>? )
+	<dfn>hsla()</dfn> = hsla( <<hue>>, <<percentage>>, <<percentage>>, <<alpha-value>>? )
 	<dfn>&lt;hue></dfn> = <<number>> | <<angle>>
 	</pre>
 
@@ -743,7 +746,7 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	but this specification does not define how to do so.
 
 	The final argument specifies the alpha channel of the color.
-	It's interpreted identically to the fourth argument of the ''rgba()'' function.
+	It's interpreted identically to the fourth argument of the ''rgb()'' function.
 	If omitted, it defaults to ''100%''.
 
 	<div class='example'>
@@ -771,6 +774,10 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 		</pre>
 	</div>
 
+	Note: For legacy reasons, the ''hsl()'' function and ''hsla()'' functions both exist,
+	but are aliases of each other.
+	Both of them allow specifying an HSL color with an optional alpha value.
+
 
 <h3 id='hsl-to-rgb'>
 Converting HSL colors to RGB colors</h3>