Add note about case-insensitivity of properties on StylePropertyMapReadOnly.

Author: nainar

css-typed-om/Overview.bs

@@ -217,6 +217,8 @@ is initialized differently depending on where the {{CSSStyleValue}} is used
 [[#declared-stylepropertymap-objects]], and
 [[#inline-stylepropertymap-objects]]).
 
+All properties stored on the [=property model=] in {{StylePropertyMapReadOnly}} are case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent).
+
 <div class='note'>
 The sequence of {{CSSStyleValue}}s associated with a property do
 not represent multiple successive definitions of that property's value.
@@ -323,8 +325,6 @@ Issue(268): stringification
 
 Issue(276): interaction with custom properties
 
-Issue(309): should StylePropertyMaps be case sensitive?
-
 Issue(310): consider using properties in addition to get/set
 
 Computed {{StylePropertyMapReadOnly}} objects {#computed-stylepropertymapreadonly-objects}