[cssom] Use nicer serialization for non-opaque colors and avoid excessive decimals in <number>

Author: Simon Pieters

cssom/Overview.bs

@@ -2590,7 +2590,7 @@ depends on the component, as follows:
  <li>The string "<code lt>, </code>".
  <li>The shortest base-ten serialization of the color's blue component.
  <li>The string "<code lt>, </code>".
- <li>The shortest serialization of the &lt;number&gt; that denotes the color's alpha component.
+ <li>The serialization of the color's alpha component as an &lt;alphavalue>.
  <li>The string "<code lt>)</code>".
  </ol>
 
@@ -2637,6 +2637,23 @@ depends on the component, as follows:
    <dd>The color is <code lt>rgba(0, 0, 0, 0)</code>.
  -->
 
+ <dt>&lt;alphavalue>
+ <dd>
+  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 &lt;number>.
+  </ol>
+
+  Otherwise, return the result of serializing the given value as a &lt;number>.
+
  <dt>&lt;counter>
  <dd>
   The return value of the following algorithm:
@@ -2684,11 +2701,12 @@ depends on the component, as follows:
 
  <dt>&lt;number>
  <dd>
-  A base-ten number using digits 0-9 (U+0030 to U+0039) in the shortest form possible, using "<code lt>.</code>" to separate decimals (if any), preceded
-  by "<code lt>-</code>" (U+002D) if it is negative.
+  A base-ten number using digits 0-9 (U+0030 to U+0039) in the shortest form possible,
+  using "<code lt>.</code>" to separate decimals (if any),
+  rounding the value if necessary to not produce more than 6 decimals,
+  preceded by "<code lt>-</code>" (U+002D) if it is negative.
 
-  Note: JavaScript's <code lt>ToString</code> algorithm cannot be used since it can serialize numbers using an exponent, which would not round-trip
-  in CSS.
+  Note: scientific notation is not used.
 
  <dt>&lt;percentage>
  <dd>The &lt;number> component serialized as per &lt;number> followed