[css-timing] Add section on serialization

Author: birtles

css-timing/Overview.bs

@@ -313,3 +313,24 @@ The syntax for specifying a [=timing function=] is as follows:
   ''linear'' |
   <<cubic-bezier-timing-function>> |
   <<step-timing-function>></div>
+
+Serialization {#serializing-a-timing-function}
+-------------
+
+Timing functions are serialized using the common serialization patterns
+defined in [[CSSOM]] with the following additional requirements:
+
+*   The keyword values ''ease'', ''linear'', ''ease-in'', ''ease-out'',
+    and ''ease-in-out'' are serialized as-is, that is, they are
+    <em>not</em> converted to the equivalent ''cubic-bezier()''
+    function before serializing.
+
+*   Step timing functions, whether they are specified using the
+    ''steps()'' function or either of the ''step-start'' or ''step-end''
+    keywords, are serialized as follows:
+
+    1.   If the <var>step position</var> is ''end'', serialize
+         as <a lt="steps()" function>steps(&lt;integer&gt;)</a>.
+
+    2.   Otherwise, serialize as <a lt="steps()"
+         function>steps(&lt;integer&gt;, start)</a>.