Whoops, use 'custom property name string' uniformly.

Author: tabatkins

css-typed-om/Overview.bs

@@ -332,7 +332,7 @@ probably in an appendix.
     when called on a {{StylePropertyMap}} |this|,
     must perform the following steps:
 
-    1. If |property| does not start with two dashes (U+002D HYPHEN),
+    1. If |property| is not a [=custom property name string=],
         let |property| be |property| [=ASCII lowercased=].
 
     2. If |property| is not a [=supported property name=],
@@ -351,7 +351,7 @@ probably in an appendix.
     when called on a {{StylePropertyMap}} |this|,
     must perform the following steps:
 
-    1. If |property| does not start with two dashes (U+002D HYPHEN),
+    1. If |property| is not a [=custom property name string=],
         let |property| be |property| [=ASCII lowercased=].
 
     2. If |property| is not a [=supported property name=],
@@ -369,7 +369,7 @@ probably in an appendix.
     when called on a {{StylePropertyMap}} |this|,
     must perform the following steps:
 
-    1. If |property| does not start with two dashes (U+002D HYPHEN),
+    1. If |property| is not a [=custom property name string=],
         let |property| be |property| [=ASCII lowercased=].
 
     2. If |property| is not a [=supported property name=],