Define list-valued and single-valued, and slightly rewrite the note about this as well. Add issue for explicitly listing which properties are which.

Author: tabatkins

css-typed-om/Overview.bs

@@ -223,14 +223,33 @@ This list is initialized differently depending on where the {{CSSStyleValue}} is
 
 All properties stored on the [=contained properties map=] in {{StylePropertyMapReadOnly}} are [=ASCII case-insensitive=].
 
-<div class='note'>
-    The sequence of {{CSSStyleValue}}s associated with a property 
-    do not represent multiple successive definitions of that property's value.
-    Instead, sequences represent values associated with [=list-valued properties=].
+Some CSS properties are <dfn export local-lt="list-valued">list-valued properties</dfn>,
+such as 'background-image' or 'animation';
+their value is a list of parallel grammar terms,
+almost always comma-separated
+(the only exceptions are certain legacy properties like 'counter-reset'),
+indicating multiple distinct "values" interpreted in the same way.
+Other properties,
+such as 'color',
+are <dfn export local-lt="single-valued">single-valued properties</dfn>;
+they take only a single (possibly complex) value.
+
+Issue: Define precisely which properties are list-valued and which aren't,
+probably in an appendix.
 
-    This approach allows single-valued properties to become list-valued in the future 
-    without breaking code that relies on calling {{StylePropertyMapReadOnly/get()}} 
-    and/or {{StylePropertyMap/set()}} for those properties.
+<div class='note'>
+    There are multiple examples of CSS properties 
+    that have transitioned from being [=single-valued=] to [=list-valued=].
+    To ensure that code written at a time when a property was [=single-valued=]
+    does not break when it becomes [=list-valued=] in the future,
+    the {{CSSPropertyMap}} is a <b>multi-map</b>;
+    it stores <em>list</em> of values for each key,
+    but allows you to interact with it as if there was only a single value for each key as well.
+
+    This means that multiple values for a single property in a {{CSSPropertyMap}}
+    do not represent multiple successive definition of that property's value;
+    instead, they represent multiple comma-separated sub-values in a single property value,
+    like each "layer" in a 'background-image' property.
 </div>
 
 The <dfn method for=StylePropertyMap>append(DOMString <var>property</var>,