[css-color-hdr] C;arified that SDR is an HDR headroom of 0 stops.

Author: svgeesus

css-color-hdr/Overview.bs

@@ -118,6 +118,7 @@ Introducing Headroom {#introducing-headroom}
 
 	It is typically expressed in photographic <strong>stops</strong>,
 	where one stop means a doubling of luminance.
+	Thus, standard dynamic range (SDR) has an HDR headroom of 0 stops.
 
 <div class="example" id="ex-low-headroom">
 	For example, a low-end HDR display