Add description for StylePropertyMapReadOnly.get() (w3c#441)

* Add description for StylePropertyMapReadOnly.get()

* Ensure that the get() function works for list-valued and non-list-valued properties

* Only return the first entry in a list of values for each property

Author: nainar

css-typed-om/Overview.bs

@@ -290,8 +290,16 @@ The <dfn method for=StylePropertyMap>append(DOMString <var>property</var>,
 <div algorithm>
     To <dfn>get a value from a StylePropertyMap</dfn>, run these steps:
 
-    Issue(148): Write this
+    1. If |property| does not start with two dashes (U+002D HYPHEN),
+        let |property| be |property| [=ASCII lowercased=].
+
+    2. If |property| is not a [=supported property name=],
+        [=throw=] a {{TypeError}} and exit this algorithm.
 
+    3. If {{StylePropertyMap}}’s [=property model=] contains an entry for |property|,
+        return the first value found in that entry.
+    
+    4. Else, return `null`.
 </div>
 
 <div algorithm>