Administration: Remove the xmlns attribute on the <html> tag.

The attribute is specific to XHTML and is not needed in HTML5.

Props audrasjb, diddledan, hommealone, joyously, mukesh27, valentinbora, peterwilsoncc, SergeyBiryukov.
Fixes #49126.

git-svn-id: https://develop.svn.wordpress.org/trunk@48126 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3426,7 +3426,7 @@ function _default_wp_die_handler( $message, $title = '', $args = array() ) {
 		}
 		?>
 <!DOCTYPE html>
-<html xmlns="http://www.w3.org/1999/xhtml" <?php echo $dir_attr; ?>>
+<html <?php echo $dir_attr; ?>>
 <head>
 	<meta http-equiv="Content-Type" content="text/html; charset=<?php echo $parsed_args['charset']; ?>" />
 	<meta name="viewport" content="width=device-width">

src/wp-includes/ms-deprecated.php

@@ -91,7 +91,7 @@ function graceful_fail( $message ) {
 	$message = apply_filters( 'graceful_fail', $message );
 	$message_template = apply_filters( 'graceful_fail_template',
 '<!DOCTYPE html>
-<html xmlns="http://www.w3.org/1999/xhtml"><head>
+<html><head>
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 <title>Error!</title>
 <style type="text/css">

src/wp-includes/theme-compat/header.php

@@ -16,7 +16,7 @@ _deprecated_file(
 );
 ?>
 <!DOCTYPE html>
-<html xmlns="http://www.w3.org/1999/xhtml" <?php language_attributes(); ?>>
+<html <?php language_attributes(); ?>>
 <head>
 <link rel="profile" href="http://gmpg.org/xfn/11" />
 <meta http-equiv="Content-Type" content="<?php bloginfo( 'html_type' ); ?>; charset=<?php bloginfo( 'charset' ); ?>" />