[css-color-hdr] add accessibility consideration w3c#553

Author: svgeesus

css-color-hdr/Overview.bs

@@ -551,3 +551,7 @@ Issue: Do the PQ-based colorspaces require tone-mapping (OOTF) for display in di
 <h2 class=no-num id=priv-sec>Privacy and Security Considerations</h2>
 
 Issue: Make some considerations about privacy and security.
+
+<h2 class="no-num" id="a11y">Accessibility Considerations</h2>
+
+Some indivduals may have a sensitivity to very bright colors, so user agents should provide a mechanism to limit the maximum luminance at user option. The toe and knee procedure in section 5.4.1 <em>Mapping to display with limited brightness range</em> of [[Rec.2390]] is suggested as suitable.