Refactor out value type-checking logic from CSSStyleValue.append to fix issue w3c#145 (w3c#429)

* refactor out value type-checking logic from CSSStyleValue.append to fix issue w3c#145

* Remove issue from spec

* Fix up algorithm referencing

Author: nainar

css-typed-om/Overview.bs

@@ -250,18 +250,23 @@ The <dfn method for=StylePropertyMap>append(DOMString <var>property</var>,
 
     5. Let |values to append| be the empty list.
 
-    6. For each |value| in |values|:
+    6. For each |value| in |values| if the [=algorithm that coerces value into an appropriate type for a given property=] does not throw an error, append the returned object to |values to append|.
+
+    7. Append |values to append| to the end of |entry|’s list. 
+</div>
+
+<div algorithm>
+    This section describes the <dfn>algorithm that coerces value into an appropriate type for a given property</dfn>, or fails and throws a {{TypeError}}:
         : If |value| is a {{CSSStyleValue}},
         :: If |value| does not match the grammar of a [=list-valued property iteration=] of |property|,
             [=throw=] a {{TypeError}} and exit this algorithm.
-            Otherwise, append |value| to the end of |values to append|.
+            Otherwise, return the |value|.
         : If |value| is a {{DOMString}},
-        ::    [=Parse a CSSStyleValue=] with property |property| and value |value|.
+        ::    [=Parse a CSSStyleValue=] with property |property| and value |value| and return the resulting |value|.
             If the result is null,
             [=throw=] a {{TypeError}} and exit this algorithm.
-            Otherwise, append each [=list-valued property iteration=] in the result to the end of |values to append|.
-
-    7. Append |values to append| to the end of |entry|’s list.
+            Otherwise, append each [=list-valued property iteration=] in the result to a |values to append| object
+            and return |values to append|.
 </div>
 
 <div algorithm>
@@ -309,8 +314,6 @@ the [=property model=]. This list of properties is sorted in the following manne
 
 [=computed StylePropertyMap=], [=declared StylePropertyMap=] and [=inline StylePropertyMap=] are all live objects: the attributes and methods on these objects must operate on the actual underlying data, not a snapshot of the data.
 
-Issue(145): should refactor out value type-checking, as it'll be needed by the rest of the setters too
-
 Issue(148): add detailed descriptions of the rest of the methods on {{StylePropertyMap}}
 
 Issue(268): stringification