[css-scrollbars] Improve wording around minimumm thickness of scrollbars

See #3315

css-scrollbars-1/Overview.bs

@@ -194,13 +194,16 @@ the desired thickness of an element’s scrollbars.
 <dd>Implementations must use the default platform scrollbar width.
 </dd>
 <dt><dfn>thin</dfn></dt>
-<dd>Implementations should use thinner scrollbars than ''scrollbar-width/auto'' when applicable.
+<dd>Implementations should use thinner scrollbars than ''scrollbar-width/auto''.
 This may mean a thin variant of scrollbar provided by the platform,
 or a custom scrollbar thinner than the default platform scrollbar.
+The scrollbar should nonetheless remain wide enough to be usable.
+(Implementers may wish to consult
+<a href="https://www.w3.org/TR/WCAG21/#target-size">WCAG 2.1 SC 2.5.5 Target Size</a>. [[WCAG21]])
 
-Note: Some platforms may only have a tiny scrollbar by default
-which cannot be or makes no sense to make thinner.
-In that case, implementations may treat this value as ''scrollbar-width/auto''.
+Note: Some platforms <del>may</del> only have a tiny scrollbar by default
+which cannot be reasonably made thinner.
+In such cases, this value will behave as ''scrollbar-width/auto''.
 </dd>
 <dt><dfn>none</dfn></dt>
 <dd>implementations must not display any scrollbar, however the element's scrollability is not affected.
@@ -209,10 +212,6 @@ In that case, implementations may treat this value as ''scrollbar-width/auto''.
 
 UAs must apply the 'scrollbar-width' value set on the root element to the viewport.
 
-UAs should enforce a minimum actual size of scrollbar width per 
-<a href="https://www.w3.org/TR/WCAG21/#target-size">WCAG 2.1 SC 2.5.5 Target Size</a> 
-[[WCAG21]].
-
 Authors that use ''scrollbar-width/none'' should provide an alternative/equivalent visual hint that 
 scrolling is possible and there is more content. 
 For situations where an element is going to be scrolled by other means instead of direct user manipulation