onment
	 *
	 * When home_url and site_url are not the same, the home_url should be used.
	 * This is because the XSL needs to be served from the same domain, protocol and port
	 * as the XML file that is loading it.
	 *
	 * @return string The XSL URL that needs to be used.
	 */
	protected function get_xsl_url() {
		if ( home_url() !== site_url() ) {
			return home_url( 'main-sitemap.xsl' );
		}

		/*
		 * Fallback to circumvent a cross-domain security problem when the XLS file is
		 * loaded from a different (sub)domain.
		 */
		if ( strpos( plugins_url(), home_url() ) !== 0 ) {
			return home_url( 'main-sitemap.xsl' );
		}

		return plugin_dir_url( WPSEO_FILE ) . 'css/main-sitemap.xsl';
	}
}