Cater for RangeError caused by dividing by 0 (w3c#589)

* Cater for RangeError caused by dividing by 0

Fixes w3c#575

* Indent properly

Author: nainar

css-typed-om/Overview.bs

@@ -878,10 +878,12 @@ The following are the arithmetic operations you can perform on dimensions:
     1. If |this| is a {{CSSMathInvert}} object,
         return |this|’s {{CSSMathInvert/value}} internal slot.
 
-    2. If |this| is a {{CSSUnitValue}} object with {{CSSUnitValue/unit}} internal slot set to "number",
-        return a new {{CSSUnitValue}}
-        with the {{CSSUnitValue/unit}} internal slot set to "number",
-        and a {{CSSUnitValue/value}} internal slot set to the inverse of |this|’s.
+    2. If |this| is a {{CSSUnitValue}} object with {{CSSUnitValue/unit}} internal slot set to "number":
+        1. If |this|’s {{CSSUnitValue/value}} internal slot is set to 0,
+            [=throw=] a {{RangeError}}.
+        2. Else return a new {{CSSUnitValue}}
+            with the {{CSSUnitValue/unit}} internal slot set to "number",
+            and a {{CSSUnitValue/value}} internal slot set to the inverse of |this|’s.
 
     3. Otherwise,
         return a new {{CSSMathInvert}} object