[css-scrollbars] Ensure sufficient contrast

UAs must ensure sufficient contrast themselves if they're not showing
the scrollbar track.

See #9855

css-scrollbars-1/Overview.bs

@@ -292,6 +292,39 @@ Value Definitions</h3>
 	Note: When a user interacts with a scrollbar (e.g. hovering or activating),
 	implementations may alter which scrollbar colors apply to which scrollbar parts.
 
+	If the user agent displays the scrollbar without the [=track=],
+	they must ensure reasonable contrast between the [=thumb=] and what is behind it.
+	However, if the author set the color of the [=track=] to be fully transparent,
+	the user agent may assume that the author has provided for sufficient contrast
+	even with an invisible track.
+
+	Note: This is typically relevant in the case of [=overlay scrollbars=],
+	either on platforms where the [=track=] is never displayed,
+	or on platforms where it is only displayed
+	when the scrollbar is being interacted with
+	but not when scrolling with the mouse wheel or the track-pad.
+	This does not apply in cases where the entire scrollbar is hidden
+	as is often the case with [=overlay scrollbars=]
+	while no scroll operation is ongoing
+	<em>and</em> when the scrollbar is not being interacted with either.
+
+	<div class=note>
+		Note: The user agent is free to achieve this by any of a variety of means,
+		depending on the platform,
+		on design choices,
+		or on what is practical to implement,
+		such as:
+
+		* Doing nothing if it can determine that contrast is already sufficient
+		* Doing nothing if the design and geometry of the thumb ensures that some part of it will always be visible regardless of what is behind it
+		* Adding an outline, shadow, or glow around the [=thumb=] to provide contrast (possibly using the [=track=] color as the contrasting color, or possibly using a different shade of the [=thumb=] color)
+		* Adding some texture or geometry to the [=thumb=] so that it's color isn't uniform
+		* Drawing the [=track=] anyway in cases where not doing so yields insufficient contrast
+		* Replacing the [=thumb=] color with the [=track=] color when that provides better contrast
+		* Altering the luminosity of the specified [=thumb=] color while preserving its hue
+		* Disregarding the specified [=thumb=] color altogether and use automatically determined alternative
+	</div>
+
 <h3 id=color-compat>
 Interaction with non-standard features</h3>