[css-scrollbars] Define interaction between scrollbar-*, and non-standard features

Closes w3c#9856

Author: frivoal

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>