Index: src/wp-includes/css/editor.css
===================================================================
--- src/wp-includes/css/editor.css	(revision 37741)
+++ src/wp-includes/css/editor.css	(working copy)
@@ -1758,11 +1758,7 @@
 }
 
 div.wp-link-preview a.wplink-url-error {
-	color: #a00;
-}
-
-div.wp-link-preview a.wplink-url-error:hover {
-	color: #f00;
+	color: #dc3232;
 }
 
 div.wp-link-input {
Index: src/wp-includes/js/tinymce/skins/wordpress/wp-content.css
===================================================================
--- src/wp-includes/js/tinymce/skins/wordpress/wp-content.css	(revision 37741)
+++ src/wp-includes/js/tinymce/skins/wordpress/wp-content.css	(working copy)
@@ -212,6 +212,18 @@
 	position: relative;
 }
 
+a[data-wplink-url-error]:before {
+	content: '';
+	display: block;
+	position: absolute;
+	top: -2px;
+	right: -2px;
+	bottom: -2px;
+	left: -2px;
+	outline: 2px dotted #fff;
+	z-index: -1;
+}
+
 /**
  * WP Views
  */