Most browsers automatically fill the content of input elements of type 'password' when this password has already been provided in the past.
Imagine that user B takes control of a machine belonging to a user A. Accessing a secured web site as user A is trivial for user B if form input elements are automatically filled in by the browser on the site's login page.
For HTML5:
<input type="password" />
For HTML5:
<input type="password" autocomplete="off" />