[css-scrollbars] ensure usability of scrollbars (#6675)

css-overflow-3/Overview.bs

@@ -767,14 +767,14 @@ Reserving space for the scrollbar: the 'scrollbar-gutter' property</h2>
 
 	Scrollbars which by default are placed over the content box
 	and do not cause <a>scrollbar gutters</a> to be created
-	are called <dfn>overlay scrollbars</dfn>.
+	are called <dfn export>overlay scrollbars</dfn>.
 	Such scrollbars are usually partially transparent, revealing the content behind them if any.
 	Their appearance and size may vary
 	based on whether and how the user is interacting with them.
 
 	Scrollbars which are always placed in a <a>scrollbar gutter</a>,
 	consuming space when present,
-	are called <dfn>classic scrollbars</dfn>.
+	are called <dfn export>classic scrollbars</dfn>.
 	Such scrollbars are usually opaque.
 
 	Whether <a>classic scrollbars</a> or <a>overlay scrollbars</a> are used,

css-scrollbars-1/Overview.bs

@@ -223,10 +223,19 @@ wider or narrower than is typical.
 <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.
+The scrollbar must 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: User agents can use various strategies
+to ensure the usability of narrow scrollbars.
+For instance, in the case of <a spec=css-overflow-3>overlay scrollbars</a>,
+they can dynamically enlarge the scrollbar
+in response to a user attempting to interact with it.
+User agents on devices with touch screens
+can also adjust how they interpret finger taps
+to facilitate interacting with visually small touch targets.
+
 User agents may disregard this value and treat it as ''scrollbar-width/auto'',
 for instance when the user has indicated discomfort
 for thin scrollbars through some UA or OS setting.
@@ -294,7 +303,8 @@ This appendix is <em>informative</em>.
 <h3 class="no-num" id="changes-since-2021-08-05">
 Changes from the <a href="https://www.w3.org/TR/2021/WD-css-scrollbars-1-20210805/">2021-08-05 Working Draft</a></h3>
 
-None yet.
+	* Switched "should" to a "must" with regards to accessibility of narrow scrollbars.
+		(see <a href="https://github.com/w3c/csswg-drafts/issues/6675">Issue 6675</a>)
 
 <h3 class="no-num" id="changes-since-2018-09-25">
 Changes from the <a href="https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/">2018-09-25 First Public Working Draft</a></h3>