[css-color-5] Added accessibility considerations section

Author: svgeesus

css-color-5/Overview.bs

@@ -3242,7 +3242,19 @@ and thus do not constitute an increased security risk.
 
 No new privacy considerations have been reported on this specification.
 
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+<h2 id="a11y-sec">Accessibility Considerations </h2>
 
+This specification adds a way to ensure adequate contrast for text whose background is a user-specified color, including dynamic colors.
 
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