[css-color-4] Missing in rgb serializes as 0, fix #6959

Author: svgeesus

css-color-4/Overview.bs

@@ -1059,13 +1059,18 @@ and there shall be no thousands separator.
 	(depending on whether the alpha is exactly 1, or not),
 	with lowercase letters for the function name.
 
+	During serialization,
+	any [=missing=] values
+	are converted to 0.
+
 	For compatibility, the sRGB component values
 	are serialized in <<number>> form, not <<percentage>>).
 	Also for compatibility,
 	the component values are serialized in base 10,
 	with a range of [0-255], regardless of
 	the bit depth with which they are stored.
 
+
 	<a href="#serializing-alpha-values">As noted earlier</a>,
 	unitary alpha values are not explicitly serialized.
 	Also, for compatibility, if the alpha is exactly 1,
@@ -1075,7 +1080,6 @@ and there shall be no thousands separator.
 	with an explicit alpha value.
 
 
-
 	For compatibility,
 	the legacy form with comma separators is used;
 	exactly one ASCII space follows each comma.
@@ -1084,6 +1088,7 @@ and there shall be no thousands separator.
 	from the alpha value.
 
 
+
 	<div class="example" id="ex-rgb-ser-int-rgba">
 		<p>For example, the serialized value of</p>
 		<pre class="lang-css"><span class="swatch" style="--color: rgba(29, 164, 192, 0.95)"></span> rgb(29 164 192 / 95%)</pre>