[css-color-4] minimum bits for serialization, closes #5667

Author: svgeesus

css-color-4/Overview.bs

@@ -844,12 +844,46 @@ and there shall be no thousands separator.
 	The precision with which ''color()'' component values are retained,
 	and thus the number of significant figures in the serialized value,
 	is not defined in this specification,
-	but for RGB and CMYK colorspaces must at least be sufficient
+	but for sRGB and CMYK colorspaces must at least be sufficient
 	to round-trip values
 	with eight bit precision;
 	this will result in at least two decimal places
 	unless trailing zeroes have been omitted.
-	(16bit, or half float, is recommended for internal storage).
+
+	For the predefined colorspaces other than sRGB,
+	which have wider gamuts and thus require more bits to avoid banding,
+	the minimum precision for round-tripping is as follows:
+
+	<table class="data">
+		<tr><th>Colorspace</th><th>Minimum bits</th></tr>
+		<tr>
+			<td>''display-p3''</td>
+			<td>10</td>
+		</tr>
+		<tr>
+			<td>''a98-rgb''</td>
+			<td>10</td>
+		</tr>
+		<tr>
+			<td>''prophoto-rgb''</td>
+			<td>12</td>
+		</tr>
+		<tr>
+			<td>''rec2020''</td>
+			<td>12</td>
+		</tr>
+		<tr>
+			<td>''xyz''</td>
+			<td>16</td>
+		</tr>
+		<tr>
+			<td>''lab''</td>
+			<td>16</td>
+		</tr>
+	</table>
+
+	(16bit, half-float, or float <em>per component</em>
+	is recommended for internal storage).
 	Values must be <a href="https://drafts.csswg.org/css-values-4/#combine-integers">rounded towards +∞</a>, not truncated.
 
 	<a href="#serializing-alpha-values">As noted earlier</a>,