Index: wp-includes/js/mediaelement/wp-mediaelement.css
===================================================================
--- wp-includes/js/mediaelement/wp-mediaelement.css	(revision 24946)
+++ wp-includes/js/mediaelement/wp-mediaelement.css	(working copy)
@@ -8,4 +8,8 @@
 
 .mejs-controls .mejs-time-rail .mejs-time-current {
 	background: #d54e21;
-}
\ No newline at end of file
+}
+
+.me-cannotplay {
+	width: auto !important;
+}
Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 24946)
+++ wp-includes/media.php	(working copy)
@@ -910,6 +910,7 @@
 		'loop'     => $loop,
 		'autoplay' => $autoplay,
 		'preload'  => $preload,
+		'style'    => 'width: 100%',
 	);
 
 	// These ones should just be omitted altogether if they are blank