Merge pull request #2992 from upsuper/scrollbar-thin

[css-scrollbars-1] Loosen requirement for scrollbar-width:thin

Author: tantek

css-scrollbars-1/Overview.bs

@@ -143,8 +143,13 @@ This property allows the author to set the maximum thickness of an element’s s
 <dd>implementations must use the default platform scrollbar width.
 </dd>
 <dt><dfn>thin</dfn></dt>
-<dd>implementations must use either a thin scrollbar width variant on platforms that provide that option, 
-or display a scrollbar thinner than the default platform scrollbar width.
+<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.
+
+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.
 </dd>
 <dt><dfn>none</dfn></dt>
 <dd>implementations must not display any scrollbar, however the element's scrollability is not affected.