Merge pull request w3c#4694 from w3c/tantek-patch-1

[css-scrollbars-1] Add suggestions (lightly edited) to resolve w3c#3315. Per @fantasai point that it is better than existing text, and leave issue open for further review/tweaking: w3c#3315 (comment)

Author: tantek

css-scrollbars-1/Overview.bs

@@ -140,8 +140,17 @@ if the corresponding part do not exist on the underlying platform.
 When using scrollbar-color property with specific color values,
 authors should ensure the specified colors have enough contrast between them.
 For keyword values, UAs should ensure the colors they use have enough contrast.
-See <a href="https://www.w3.org/TR/WCAG20-TECHS/G183.html">
-Techniques for WCAG 2.0: G183: Using a contrast ratio of 3:1 […]</a> [[WCAG20]].
+UAs should ensure that their actual color values for 
+scrollbar-color: light and scrollbar-color: dark 
+should provide sufficient contrast with the element’s background
+(i.e. scrollbar-color: light on a light background and scrollbar-color: dark on a dark background 
+should always have sufficient contrast)
+See 
+<a href="https://www.w3.org/TR/WCAG21/#non-text-contrast">WCAG 2.1 SC 1.4.11 Non-text Contrast</a> 
+[[WCAG21]]. 
+UAs may ignore these contrast requirements based on explicit user preferences 
+(for example, when users choose a configuration option/setting that 
+always ensures a particular scrollbar color / use of system default scrollbars).
 
 Note: when a user interacts with a scrollbar (e.g. hovering or activating), 
 implementations may alter which scrollbar colors apply to which scrollbar parts.
@@ -197,6 +206,15 @@ 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 
+(e.g. programmatically), authors should use overflow: hidden instead of scrollbar-width: none.
+
 Note: Unlike 'overflow' (and overflow-*) properties, 
 a scrollbar-width value set on the HTML body element is not propagated to the viewport.
 
@@ -209,8 +227,12 @@ This appendix is <em>informative</em>.
 
 <p>
 Thanks to the use-cases, prototyping, implementation, and feedback from
-<span class="h-card">Tab Atkins</span> and
-<span class="h-card">Xidorn Quan</span>.
+<a class="h-card" href="https://xanthir.com/">Tab Atkins</a> and
+<a class="h-card" href="https://www.upsuper.org/">Xidorn Quan</a>.
+Thanks to accessibility review and contributions 
+(<a href="https://github.com/w3c/csswg-drafts/issues/3315">#3315</a>) 
+from 
+<a class="h-card" href="https://www.splintered.co.uk">Patrick H. Lauke</a>.
 </p>
 
 <h2 class="no-num" id="changes">Appendix B. Changes</h2>
@@ -221,9 +243,14 @@ This appendix describes changes from the
 <a href="https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/">First Public Working Draft (FPWD) of 25 Sep 2018</a>.
 <ul>
 <li><a href="https://github.com/w3c/csswg-drafts/issues/3237">#3237</a>: 
-    scrollbar-color computed value changed to: specified keyword or two computed colors</li>
+    scrollbar-color computed value changed to: specified keyword or two computed colors
+</li>
 <li><a href="https://github.com/w3c/csswg-drafts/issues/4693">#4693</a>: 
-    Clarified scope: styling scrollbar controls themselves.</li>
+    Clarified scope: styling scrollbar controls themselves, no layout or scrollability.
+</li>
+<li><a href="https://github.com/w3c/csswg-drafts/issues/3315">#3315</a>: 
+    More and updated accessibility considerations for scrollbar-color and scrollbar-width.
+</li>
 </ul>
 
 <h2 class="no-num" id="security-privacy-considerations">Appendix C. Considerations for Security and Privacy</h2>