[cssom] Serialize :lang() arguments as strings. https://lists.w3.org/Archives/Public/www-style/2015Feb/0345.html

Author: Simon Pieters

cssom/Overview.bs

@@ -645,13 +645,14 @@ finally return <var>s</var>:
 
   Otherwise, append "<code lt>:</code>" (U+003A), followed by the name of
   the pseudo-class, followed by "<code lt>(</code>" (U+0028), followed by the
-  value of the pseudo-class argument determined as per below, followed by
+  value of the pseudo-class argument(s) determined as per below, followed by
   "<code lt>)</code>" (U+0029), to <var>s</var>.
 
   <dl class="switch">
    <dt><code lt>:lang()</code>
-   <dd>The <span lt="serialize an identifier">escaped</span>
-   value.
+   <dd>The value of each argument <span lt="serialize a string">string escaped</span>,
+   preserving relative order,
+   separated by "<code lt>, </code>" (U+002C U+0020).
 
    <dt><code lt>:nth-child()</code>
    <dt><code lt>:nth-last-child()</code>
@@ -3200,6 +3201,7 @@ Acknowledgments {#acknowledgments}
 
 The editors would like to thank
 Alexey Feldgendler,
+Benjamin Poulain,
 Bj&ouml;rn H&ouml;hrmann,
 Boris Zbasky,
 Brian Kardell,