[css-color-4] Collected together and clarified serialization of opacity values into one section #10426

Author: svgeesus

css-color-4/Overview.bs

@@ -6723,6 +6723,36 @@ Serializing other colors</h3>
 
 	The serialized form of ''currentColor'' is the string "currentcolor".
 
+
+<h2 id='serializing-opacity-values'>
+Serializing <<opacity-value>></h2>
+
+	This applies to the ''opacity'' property.
+
+	The specified given opacity value is serialized
+	as a <<number>>, not a <<percentage>>.
+
+	This <<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.
+
+	Opacity values outside the range [0,1]
+	are preserved, without clamping, in the serialized specified value.
+
+	The precision with which opacity values are retained,
+	and thus the number of decimal places in the serialized value,
+	is not defined in this specification,
+	but must at least be sufficient
+	to round-trip integer percentage values.
+	Thus, the serialized value must contain
+	at least two decimal places
+	(unless trailing zeroes have been removed).
+	Values must be
+	<a href="https://drafts.csswg.org/css-values-4/#combine-integers">rounded 
+	towards +∞</a>, not truncated.
+
+
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