[cssom] Specify the serialization of <ratio>.

Part of w3c#4282

Author: emilio

cssom-1/Overview.bs

@@ -2701,6 +2701,11 @@ depends on the component, as follows:
  <dd>The resolution in dots per <a lt=px value>CSS pixel</a> serialized as per
  &lt;number> followed by the literal string "<code>dppx</code>".
 
+ <dt>&lt;ratio>
+ <dd>The numerator serialized as per &lt;number>
+     followed by the literal string "<code> / </code>",
+     followed by the denominator serialized as per &lt;number>.
+
  <dt>&lt;shape>
  <dd>
   The return value of the following algorithm: