diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index c4c44a0c69..7aada19c9c 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -149,7 +149,7 @@ function wp_authenticate_username_password( $user, $username, $password ) {
 	if ( ! $user ) {
 		return new WP_Error(
 			'invalid_username',
-			__( 'Unknown username. Check again or try your email address.' )
+			__( '<strong>Error</strong>: Unknown username. Check again or try your email address.' )
 		);
 	}
 
@@ -347,12 +347,12 @@ function wp_authenticate_application_password( $input_user, $username, $password
 		if ( is_email( $username ) ) {
 			$error = new WP_Error(
 				'invalid_email',
-				__( 'Unknown email address. Check again or try your username.' )
+				__( '<strong>Error</strong>: Unknown email address. Check again or try your username.' )
 			);
 		} else {
 			$error = new WP_Error(
 				'invalid_username',
-				__( 'Unknown username. Check again or try your email address.' )
+				__( '<strong>Error</strong>: Unknown username. Check again or try your email address.' )
 			);
 		}
 	} elseif ( ! wp_is_application_passwords_available() ) {