[css-transitions] Convert to en-US spelling.

Author: dbaron

css-transitions/Overview.bs

@@ -590,7 +590,7 @@ Starting of transitions {#starting}
         and inheriting from
         the <a>after-change style</a> of the parent.
         Note that this means the <a>after-change style</a> does not differ from
-        the <a>before-change style</a> due to newly created or cancelled CSS
+        the <a>before-change style</a> due to newly created or canceled CSS
         Animations.
       </p>
 
@@ -1192,11 +1192,11 @@ The different types of transition events that can occur are:
     <dt><dfn id=transitioncancel>transitioncancel</dfn></dt>
     <dd>
       The {{transitioncancel}} event occurs when a transition is <a
-      lt="cancel">cancelled</a>.
+      lt="cancel">canceled</a>.
 
       The {{TransitionEvent/elapsedTime}} for {{transitioncancel}} events is
       the number of seconds from the end of the transition's delay to the
-      moment when the transition was cancelled.
+      moment when the transition was canceled.
       If the transition had a negative 'transition-delay', the beginning of the
       transition is the moment equal to the absolute value of 'transition-delay'
       seconds <em>prior</em> to when the transition was actually triggered.