Do I need to use a block or form to implement login box, or can I just post to a 'login' url the user details? I have static HTML for a login box which contains a form with username and password inputs and is part of the site header, which will be on every page ie. this is not a login 'page'. 

To integrate this with the Drupal login system, can I point the form action at a certain url to post the login details and perform the login? Thereby, not requiring any Drupal backend form/block/theme coding?

Or do I need to implement my own form, or customise an existing form to achieve this? If so, why is that? What do I need from the Drupal back end to successfully login ie. hidden form fields that need to be passed with the login details etc?

I am trying to implement the site in the cleanest way possible, using my own HTML 5/ CSS 3 markup, so overriding most html so that the tonne of classes Drupal adds, and the extra unnecessary markup are scraped. So, ideally I would like to avoid having to make a login form/block as part of the back end.
You can print login block in the tpl file. No need to use username/password fields of HTML/CSS.

&lt;?php print drupal_get_form('user_login_block'); ?&gt;


For Registration :

&lt;?php print drupal_get_form('user_register_form'); ?&gt;