Make CSSNumericValue members readonly (#602)

Author: darrnshn

css-typed-om/Overview.bs

@@ -1570,7 +1570,7 @@ are represented as {{CSSUnitValue}}s.
     [Constructor(double value, DOMString unit)]
     interface CSSUnitValue : CSSNumericValue {
         attribute double value;
-        attribute DOMString unit;
+        readonly attribute DOMString unit;
     };
 </xmp>
 
@@ -1588,21 +1588,6 @@ are represented as {{CSSUnitValue}}s.
         and its {{CSSUnitValue/unit}} set to |unit|.
 </div>
 
-<div algorithm="CSSUnitValue.unit">
-    The <dfn attribute for=CSSUnitValue>unit</dfn> attribute of a {{CSSUnitValue}} |this| must,
-    on setting a value |unit|,
-    perform the following steps:
-
-    1. If [=creating a type=] from |unit| returns failure,,
-        [=throw=] a {{TypeError}}.
-
-    2. Otherwise,
-        set |this|’s {{CSSUnitValue/unit}} internal slot to |unit|.
-
-    On reading,
-    it must return the value of |this|’s {{CSSUnitValue/unit}} internal slot.
-</div>
-
 <div algorithm>
     The <dfn>[=type=] of a {{CSSUnitValue}}</dfn>
     is the result of [=creating a type=] from its {{CSSUnitValue/unit}} internal slot.
@@ -1681,32 +1666,32 @@ interface CSSMathValue : CSSNumericValue {
 
 [Constructor(CSSNumberish... args)]
 interface CSSMathSum : CSSMathValue {
-    attribute CSSNumericArray values;
+    readonly attribute CSSNumericArray values;
 };
 
 [Constructor(CSSNumberish... args)]
 interface CSSMathProduct : CSSMathValue {
-    attribute CSSNumericArray values;
+    readonly attribute CSSNumericArray values;
 };
 
 [Constructor(CSSNumberish arg)]
 interface CSSMathNegate : CSSMathValue {
-    attribute CSSNumericValue value;
+    readonly attribute CSSNumericValue value;
 };
 
 [Constructor(CSSNumberish arg)]
 interface CSSMathInvert : CSSMathValue {
-    attribute CSSNumericValue value;
+    readonly attribute CSSNumericValue value;
 };
 
 [Constructor(CSSNumberish... args)]
 interface CSSMathMin : CSSMathValue {
-    attribute CSSNumericArray values;
+    readonly attribute CSSNumericArray values;
 };
 
 [Constructor(CSSNumberish... args)]
 interface CSSMathMax : CSSMathValue {
-    attribute CSSNumericArray values;
+    readonly attribute CSSNumericArray values;
 };
 
 interface CSSNumericArray {}; // See issue below