[css-color-4] Add Accessibility Considerations

Author: svgeesus

css-color-4/Overview.bs

@@ -3183,6 +3183,7 @@ Changes</h2>
 5 November 2019</a></h3>
 
 <ul>
+	<li>Added an Accessibility Considerations section</li>
 	<li>Started to describe chroma-reduction gamut mapping rather than per-component clipping</li>
 	<li>Corrected white chromaticity for rec2020</li>
 	<li>Made device-cmyk available by @color-profile; updated CMYK to color algorithm to only use naive conversion as a last resort</li>
@@ -3304,3 +3305,17 @@ as they make it easier for a malware site
 to create user interfaces that appear to be from the system.
 However, as several system colors are now defined to be "generic",
 this risk is believed to be mitigated.
+
+Accessibility Considerations {#a11y-sec}
+========================================
+
+This specification <a href="#notes">encourages authors to not use
+	color alone</a> as a distinguuishing feature.
+
+This specification <a href="css-system-colors">encourages browsers to ensure
+	adequate contrast for specific system color foreground/background pairs</a>.
+	A harder requirement with specific AA or AAA contrast ratios was considered,
+	but since browsers are often just passing along coor choices made by the OS,
+	or selected by users (who may have particular requirements,
+	including lower contrast for migraine or epileps sufferers), the CSSWG
+	was unable to require a specific contrast level.