[css-color-4] Added steps for serializing a uint8_t alpha, from cssom-1

#9798

css-color-4/Overview.bs

@@ -5943,9 +5943,49 @@ Serializing alpha values</h3>
 	an implicit value of 1 (fully opaque) is the default.
 
 	If the alpha is any other value than 1,
-	it is explicitly included in the serialization,
-	as a <<number>>, not a <<percentage>>.
-	The value is expressed in base ten,
+	it is explicitly included in the serialization as described below.
+
+	If the value is internally represented as an integer 
+	between 0 and 255 inclusive (i.e. 8-bit unsigned integer),
+	follow these steps:
+
+	<ol>
+		<li>Let <var>alpha</var> be the given integer.
+		<li>If there exists an integer between 0 and 100 inclusive that,
+		when multiplied with 2.55 and rounded to the closest integer 
+		(rounding up if two values are equally close), equals <var>alpha</var>,
+		let <var>rounded</var> be that integer divided by 100.
+		<li>Otherwise, let <var>rounded</var> be <var>alpha</var> 
+		divided by 0.255 and rounded to the closest integer 
+		(rounding up if two values are equally close),
+		divided by 1000.
+	 	<li>Return the result of serializing <var>rounded</var> 
+		as a <<number>>.
+	</ol>
+
+	Otherwise, return the result of serializing the given value 
+	(as a <<number>>, not a <<percentage>>).
+
+	<div class="example" id="ex-alpha-255">
+		<p>
+			For example, 
+			if the alpha is stored as the 8-bit unsigned integer 237,
+			the integer 93 satisfies the criterion
+			because Math.round(93 * 2.55) is 237,
+			and so the alpha is serialized as "0.93".
+		</p>
+		<p>
+			However, 
+			if the alpha is stored as the 8-bit unsigned integer 236,
+			there is no such integer
+			(92 maps to 235 while 94 maps to 240),
+			and so since 236 ÷ 0.255 = 925.490196078
+			the alpha is serialized as "0.92549"
+			(no more than 6 figures, trailing zeroes omitted).
+		</p>
+	</div>
+
+	The <<number>> value is expressed in base ten,
 	with the "." character as decimal separator.
 	The leading zero must not be omitted.
 	Trailing zeroes must be omitted.