[css-scrollbars] Define interaction between scrollbar-*, and non-stan…

…dard features

Closes #9856

css-scrollbars-1/Overview.bs

@@ -305,6 +305,28 @@ Value Definitions</h3>
  </code></pre>
  </div>
 
+<h3 id=color-compat>
+Interaction with non-standard features</h3>
+
+ On any element or [=pseudo-element=]
+ where the [=computed value=] of this property is anything other than the initial ''scrollbar-color/auto'' value,
+ user agents must ignore any alternative non-standard means
+ for authors to influence the rendering of scrollbars,
+ <span class=non-normative>such as the ''::webkit-scrollbar'' family of [=pseudo-elements=]</span>.
+
+ <wpt>
+ scrollbar-color-001.html
+ scrollbar-color-002.html
+ scrollbar-color-003.html
+ scrollbar-color-004.html
+ scrollbar-color-005.html
+ scrollbar-color-006.html
+ scrollbar-color-007.html
+ scrollbar-color-008.html
+ scrollbar-color-009.html
+ scrollbar-color-010.html
+ scrollbar-color-dynamic-8.html
+ </wpt>
 
 <h2 id="scrollbar-width">Scrollbar Thickness: the 'scrollbar-width' property</h2>
 
@@ -473,6 +495,25 @@ Value Definitions</h3>
  Note: This specification does not define the exact position or shape of the scrollbar,
  or any animation thereof, such as fading or sliding in/out of view.
 
+<h3 id=width-compat>
+Interaction with non-standard features</h3>
+
+ On any element or [=pseudo-element=]
+ where the [=computed value=] of this property is anything other than the initial ''scrollbar-width/auto'' value,
+ user agents must ignore any alternative non-standard means
+ for authors to influence the rendering of scrollbars,
+ <span class=non-normative>such as the ''::webkit-scrollbar'' family of [=pseudo-elements=]</span>.
+
+ <wpt>
+ scrollbar-width-010.html
+ scrollbar-width-011.html
+ scrollbar-width-012.html
+ scrollbar-width-013.html
+ scrollbar-width-014.html
+ scrollbar-width-015.html
+ scrollbar-width-016.html
+ </wpt>
+
 <h2 class="no-num" id="acknowledgments">
 Appendix A. Acknowledgments</h2>