Add wording for CSSNumericValue.type() (w3c#546)

* First attempt at type() interface

* Use required long for CSSNumericType

* Make CSSNumericType members optional

* Add explanation for CSSNumericValue.type()

* Clean up CSSNumericType a bit

Author: darrnshn

css-typed-om/Overview.bs

@@ -681,6 +681,27 @@ All numeric CSS values
 are represented by subclasses of the {{CSSNumericValue}} interface.
 
 <xmp class=idl>
+    enum CSSNumericBaseType {
+        "length",
+        "angle",
+        "time",
+        "frequency",
+        "resolution",
+        "flex",
+        "percent",
+    };
+
+    dictionary CSSNumericType {
+        long length;
+        long angle;
+        long time;
+        long frequency;
+        long resolution;
+        long flex;
+        long percent;
+        CSSNumericBaseType percentHint;
+    };
+
     interface CSSNumericValue : CSSStyleValue {
         CSSNumericValue add(CSSNumberish... values);
         CSSNumericValue sub(CSSNumberish... values);
@@ -693,15 +714,12 @@ are represented by subclasses of the {{CSSNumericValue}} interface.
 
         CSSUnitValue to(DOMString unit);
         CSSMathSum toSum(DOMString... units);
-        // ??? type();
+        CSSNumericType type();
 
         static CSSNumericValue parse(DOMString cssText);
     };
 </xmp>
 
-Issue(482): Figure out how we want to represent the type of an expression in JS,
-and define the type() method accordingly.
-
 The methods on the {{CSSNumericValue}} superclass
 represent operations that all numeric values can perform.
 
@@ -1035,6 +1053,24 @@ The following are the arithmetic operations you can perform on dimensions:
         is set to |result|.
 </div>
 
+<div algorithm="CSSNumericValue.type()">
+    The <dfn method for=CSSNumericValue>type()</dfn> method
+    returns a representation of the [=type=] of |this|.
+
+    When called, it must perform the following steps:
+
+    1. Let |result| be a new {{CSSNumericType}}.
+
+    2. For each |baseType| → |power| in the [=type=] of |this|,
+        1. If |power| is not 0,
+            set |result|[|baseType|] to |power|.
+
+    3. If the [=percent hint=] of |this| is not null,
+        1. Set {{CSSNumericType/percentHint}} to the [=percent hint=] of |this|.
+
+    4. Return |result|.
+</div>
+
 <div algorithm="sum value">
     A <dfn>sum value</dfn>
     is an abstract representation of a {{CSSNumericValue}}