Bug 1567335 - Add an explicit CSS transition duration for the markup view's flash-out class. r=gl

Differential Revision: https://phabricator.services.mozilla.com/D38805

--HG--
extra : moz-landing-system : lando

devtools/client/themes/markup.css

@@ -291,7 +291,8 @@ ul.children + .tag-line::before {
 }
 
 .flash-out {
-  transition: background,color .5s;
+  transition-property: background,color;
+  transition-duration: .5s .5s;
 }
 
 .editor {