Add note about case-insensitivity of properties on StylePropertyMapReadOnly (w3c#445)

* Add note about case-insensitivity of properties on StylePropertyMapReadOnly.

* Autolink to [=ASCII case-insensitive=]

Author: nainar

css-typed-om/Overview.bs

@@ -218,6 +218,8 @@ 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 [=ASCII 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.
@@ -373,8 +375,6 @@ 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}