login_head
login_head is an action that is fired in the <head></head> section on all pages of the wp-login.php file which creates the login, registration, and lost password forms.
This hook provides no parameters. You use this hook by having your function echo output to the browser, or by having it perform background tasks. Your functions shouldn't return, and shouldn't take any parameters
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('login_head', 'your_function');
This hook was introduced in WordPress 2.1, and will not work in earlier versions.