[cssom] Add IDL CSSOMString, typedef of either USVString or DOMString

Implementations can choose one or the other.

CSSWG resolution:
w3c#1217 (comment)

Author: SimonSapin

cssom/Overview.bs

@@ -256,6 +256,41 @@ the last item. Unless otherwise specified, an empty list is serialized as the
 empty string.
 
 
+CSSOMString {#cssomstring-type}
+===============================
+
+Most strings in CSSOM interfaces use the <dfn>CSSOMString</dfn> type.
+Each implementation chooses to define it as either {{USVString}} or {{DOMString}}:
+
+<pre class=idl>
+typedef USVString CSSOMString;
+</pre>
+
+Or, alternatively:
+
+<pre class="def lang-webidl">
+typedef DOMString CSSOMString;
+</pre>
+
+<div class=note>
+	The difference is only observable from web content
+	when <a>surrogate</a> code units are involved.
+	{{DOMString}} would preserve them,
+	whereas {{USVString}} would replace them with U+FFFD REPLACEMENT CHARACTER.
+
+	This choice effectively allows implementations to do this replacement,
+	but does not require it.
+
+	This enables an implementation to use UTF-8 internally to represent strings in memory.
+	Since well-formed UTF-8 specifically disallows <a>surrogate</a> code points,
+	it effectively requires this replacement.
+
+	On the other hand,
+	implementations that internally represent strings as 16-bit <a>code units</a>
+	may prefer to avoid the cost of doing this replacement.
+</div>
+
+
 Media Queries {#media-queries}
 ==============================