Index: wp-content/themes/twentyfourteen/style.css
===================================================================
--- wp-content/themes/twentyfourteen/style.css	(revision 26653)
+++ wp-content/themes/twentyfourteen/style.css	(working copy)
@@ -89,6 +89,7 @@
 canvas,
 video {
 	display: inline-block;
+	max-width: 100%;
 }
 
 html {