Fires inside the HTML tag in the admin header.
wp-admin/includes/template.php:2657_wp_admin_html_begin() <?php /** * Fires inside the HTML tag in the admin header. * * @since 2.2.0 */ do_action( 'admin_xml_ns' ); language_attributes(); ?>><head><meta http-equiv="Content-Type" content="<?php bloginfo( 'html_type' ); ?>; charset=<?php echo get_option( 'blog_charset' ); ?>" />Introduced in 2.2.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.