[web-animations-1] Resolve ambiguous reference to transparent

Author: birtles

web-animations-1/Overview.bs

@@ -203,6 +203,7 @@ spec:dom; type:interface; text:DocumentOrShadowRoot
 spec:dom; type:interface; text:EventTarget
 spec:dom; type:interface; text:Event
 spec:dom; type:dfn; for:/; text:shadow root
+spec:css-color-4; type:value; text:transparent
 spec:css-values-3; type:type; text:<ident>
 spec:css-backgrounds-3; type:property;
     text:border-width