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.

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.
 
 <dl dfn-type="value" dfn-for="scrollbar-face-color">
 <dt><dfn>auto</dfn>
@@ -121,3 +121,34 @@ See related <a href="https://github.com/w3c/csswg-drafts/issues/1956">Issue 1956
 <!-- -->    scrollbar-track-color: Scrollbar;
 <!-- -->}</code></pre>
   </div>
+
+
+<h2 id="scrollbar-width">Scrollbar Thickness: the 'scrollbar-width' property</h2>
+
+<pre class="propdef"> 
+Name: scrollbar-width 
+Value: auto | <<length>>
+Initial: auto
+Inherited: no
+Computed value: absolute length or the keyword auto.
+Animation type: <a href="https://drafts.csswg.org/css3-transitions/#animtype-length">length</a>
+Applies to: block containers and boxes that establish a formatting context [same as elements that <a href=https://www.w3.org/TR/CSS22/visufx.html#propdef-overflow">'overflow' applies to</a>]
+Percentages: n/a
+Media: visual
+</pre>
+
+This property allows the author to set the maximum thickness of an element’s scrollbars when they are shown.
+Negative <<length>> 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.