[css-scrollbar] Add text for contrast concern of scrollbar colors

Author: upsuper

css-scrollbars-1/Overview.bs

@@ -114,6 +114,12 @@ PNG of the same in a browser that supports it currently)
 Implementations may ignore any of the colors
 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]].
+
 Note: when a user interacts with a scrollbar (e.g. hovering or activating), 
 implementations may alter which scrollbar colors apply to which scrollbar parts.