[css-variables] Clarify that custom property names just need to be case-preserved.

Author: tabatkins

css-variables/Overview.html

@@ -698,8 +698,12 @@ <h3 id=the-cssstyledeclaration-interface><span class=secno>4.1. </span>
   <h4 id=serializing-custom-props><span class=secno>4.1.1. </span>
    Serializing Custom Properties</h4>
 
-  <p> Custom property names must be serialized exactly as provided by the
-   author.
+  <p> Custom property names must be serialized with the casing as provided by
+   the author.
+
+  <p class=note> Ordinarily, property name are restricted to the ASCII range
+   and are case-insensitive, so implementations typically serialize the name
+   lowercased.
 
   <h3 id=the-cssvariablesdeclaration-interface><span class=secno>4.2. </span>
    The <a

css-variables/Overview.src.html

@@ -479,7 +479,11 @@ <h3 id='the-cssstyledeclaration-interface'>
 <h4 id='serializing-custom-props'>
 Serializing Custom Properties</h4>
 
-	Custom property names must be serialized exactly as provided by the author.
+	Custom property names must be serialized with the casing as provided by the author.
+	
+	<p class='note'>
+		Ordinarily, property name are restricted to the ASCII range and are case-insensitive,
+		so implementations typically serialize the name lowercased.
 
 <h3 id='the-cssvariablesdeclaration-interface'>
 The <code>CSSVariablesDeclaration</code> Interface</h3>