[css-overflow] Propagate scrollbar-gutter to the viewport

See w3c#6079

Author: frivoal

css-overflow-4/Overview.bs

@@ -316,6 +316,13 @@ When the <a>scrollbar gutter</a> is present but the scrollbar is not,
 or the scrollbar is transparent or otherwise does not fully obscure the <a>scrollbar gutter</a>,
 the background of the <a>scrollbar gutter</a> must be painted as an extension of the padding.
 
+As for the 'overflow' property,
+when 'scrollbar-gutter' is set on the root element,
+the user agent must apply it to the viewport instead,
+and the used value on the root element itself is ''scrollbar-gutter: auto''.
+However, unlike the 'overflow' property,
+the user agent must not propagate 'scrollbar-gutter' from the HTML <{body}> element.
+
 <div class=note>
 Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter'
 showing in which case space is reserved for a <a>classic scrollbar</a>'s <a>scrollbar gutter</a>.