minor wording consistency and grammar

Author: tantek

css-scrollbars-1/Overview.bs

@@ -155,11 +155,11 @@ This property allows the author to set the maximum thickness of an element’s s
 <dt><dfn>thin</dfn></dt>
 <dd>Implementations should use thinner scrollbars than auto when applicable.
 This may mean a thin variant of scrollbar provided by the platform,
-or a custom scrollbar thinner than native.
+or a custom scrollbar thinner than the default platform scrollbar.
 
 Note: Some platforms may only have a tiny scrollbar by default
-which doesn't make sense to be thinner.
-In that case, implementations can treat this the same as auto.
+which cannot be or makes no sense to make thinner.
+In that case, implementations may treat this value as auto.
 </dd>
 <dt><dfn>none</dfn></dt>
 <dd>implementations must not display any scrollbar, however the element's scrollability is not affected.