add scrollbar-width per issue #1958

Define a new 'scrollbar-width' property to set the maximum thickness of element scrollbars if any / when shown, per issue #1958.

Author: tantek

css-scrollbars-1/Overview.bs

@@ -73,7 +73,7 @@ These properties allow the author to set colors for various aspects of an elemen
 UAs must apply the scrollbar-* values set on the root element to the viewport.
 
 Note: Unlike 'overflow' (and overflow-*) properties, 
-scrollbar-* values set on the HTML body element are not propagated to viewport.
+scrollbar-* values set on the HTML body element are not propagated to the viewport.
 
 
 auto
@@ -121,3 +121,34 @@ See related Issue 1956
     scrollbar-track-color: Scrollbar;
 
   
+
+
+Scrollbar Thickness: the 'scrollbar-width' property
+
+ 
+Name: scrollbar-width 
+Value: auto | <>
+Initial: auto
+Inherited: no
+Computed value: absolute length or the keyword auto.
+Animation type: length
+Applies to: block containers and boxes that establish a formatting context [same as elements that 'overflow' applies to]
+Percentages: n/a
+Media: visual
+
+
+This property allows the author to set the maximum thickness of an element’s scrollbars when they are shown.
+Negative <> values are not allowed.
+
+When a user agent displays one or more scrollbars for an element, 
+the thickness of those scrollbars must be no more than the computed 'scrollbar-width' value.
+
+When scrollbar-width is 'auto', implementations must use the default platform scrollbar width.
+
+UAs must apply the scrollbar-width value set on the root element to the viewport.
+
+Note: Unlike 'overflow' (and overflow-*) properties, 
+a scrollbar-width value set on the HTML body element is not propagated to the viewport.
+
+Note: This specification does not define the exact position or shape of the scrollbar, 
+or any animation thereof, such as fading or sliding in/out of view.