Defer Javascript/CSS in WordPress

functions.php

function defer_parsing_of_js($url)
{
  if (is_admin()) return $url; //don't break WP Admin
  if (false === strpos($url, '.js')) return $url;
  if (strpos($url, 'jquery.js')) return $url;
  return str_replace(' src', ' defer src', $url);
}
add_filter('script_loader_tag', 'defer_parsing_of_js', 10);

hooks.php

function defer_parsing_of_css( $url ) {
	if ( strpos( $url, 'dist/leaflet.css' ) ) {
		return str_replace( ' href', ' async defer href', $url );
	}
	
	return $url;
}

add_filter( 'style_loader_tag', 'defer_parsing_of_css', 99 );