Set fetchpriority=low on comment-reply script

src/wp-includes/script-loader.php

@@ -1053,7 +1053,10 @@ function wp_default_scripts( $scripts ) {
 	did_action( 'init' ) && $scripts->localize( 'swfupload-handlers', 'swfuploadL10n', $uploader_l10n );
 
 	$scripts->add( 'comment-reply', "/wp-includes/js/comment-reply$suffix.js", array(), false, 1 );
-	did_action( 'init' ) && $scripts->add_data( 'comment-reply', 'strategy', 'async' );
+	if ( did_action( 'init' ) ) {
+		$scripts->add_data( 'comment-reply', 'strategy', 'async' );
+		$scripts->add_data( 'comment-reply', 'fetchpriority', 'low' ); // In Chrome this is automatically low due to the async strategy, but in Firefox and Safari the priority is normal/medium.
+	}
 
 	$scripts->add( 'json2', "/wp-includes/js/json2$suffix.js", array(), '2015-05-03' );
 	did_action( 'init' ) && $scripts->add_data( 'json2', 'conditional', 'lt IE 8' );