do_action( 'wp_head' );

function hook_javascript() {
    ?>
        <script>
            alert('Page is loading...');
        </script>
    <?php
}
add_action('wp_head', 'hook_javascript');
function hook_css() {
    ?>
        <style>
            .wp_head_example {
                background-color : #f1f1f1;
            }
        </style>
    <?php
}
add_action('wp_head', 'hook_css');