I just discovered this after searching and thought I would share. I was tasked with adding the Username and Password text to the background of the wordpress login form. The attribute 'placeholder' does this but I couldn't find any hooks that would allow me to add that attribute. You can change attributes but wp_login_form seems to filter that one out.
$('#user_login').attr( 'placeholder', 'Username' );
$('#user_pass').attr( 'placeholder', 'Password' );
Hope you find that useful. It there are better solutions, please share!!