Retrieves a list of unique hosts of all enqueued scripts and styles.

function wp_dependencies_unique_hosts() {
	global $wp_scripts, $wp_styles;

	$unique_hosts = array();

	foreach ( array( $wp_scripts, $wp_styles ) as $dependencies ) {
		if ( $dependencies instanceof WP_Dependencies && ! empty( $dependencies->queue ) ) {
			foreach ( $dependencies->queue as $handle ) {
				if ( ! isset( $dependencies->registered[ $handle ] ) ) {
					continue;
				}

				/* @var _WP_Dependency $dependency */
				$dependency = $dependencies->registered[ $handle ];
				$parsed     = wp_parse_url( $dependency->src );

				if ( ! empty( $parsed['host'] )
					&& ! in_array( $parsed['host'], $unique_hosts, true ) && $parsed['host'] !== $_SERVER['SERVER_NAME']
				) {
					$unique_hosts[] = $parsed['host'];
				}
			}
		}
	}

	return $unique_hosts;
}