Add an accessibility considerations appendix

As suggested by APA while discussing #6351

css-scrollbars-1/Overview.bs

@@ -402,3 +402,31 @@ or on overflowing elements with scrollbars in the page.</p>
 <p>No.</p>
 </li>
 </ol>
+
+<h2 class="no-num" id="accessibility-considerations">Appendix D. Considerations for accessibility</h2>
+
+This appendix is <em>informative</em>.
+
+As noted [[#scrollbar-width|in the definition of the property]],
+authors need to be mindful of the accessibility implications
+of using ''scrollbar-width: thin''.
+Scrollbars are a important piece of the user agent's interface,
+and it is not appropriate for a web site author to change their size over aesthetic considerations.
+The property is available to support cases
+where the author wants to indicate that in a cramped area of the web page
+a thin scrollbar would be a more effective use of space.
+However, ultimately, the user, through their user agent,
+needs to have the last word on such things.
+
+Using this property in such cases is preferable to
+authors building a custom thin-looking scrollbar in via script or proprietary extensions,
+because it does give the user the opportunity to override it.
+
+[=User style sheets=] do provide such an override,
+and additionally, user agents are encouraged to expose a setting
+letting users express that they do not want thin scrollbars to be used.
+
+The CSS Working Group also acknowledges the needs of some users
+to have scrollbars that are wider than is typical.
+Operating systems and user agents can offer a means to let users express that preference,
+and in such cases, CSS will honor that choice.