[css-color] optional alpha for rgb() and hsl()

Author: svgeesus

css-color/Overview.bs

@@ -171,7 +171,7 @@ The RGB functions: ''rgb()'' and ''rgba()''</h3>
 	Its syntax is:
 
 	<pre class='prod'>
-	<dfn>rgb()</dfn> = rgb( <<rgb-component>>#{3} )
+	<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>>
@@ -709,7 +709,7 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	The syntax of the ''hsl()'' function is:
 
 	<pre class='prod'>
-	<dfn>hsl()</dfn> = hsl( <<hue>>, <<percentage>>, <<percentage>> )
+	<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>