I have assigned class names to both user name & password text fields in user login block separately like this and it's working.

function mymodule_form_alter(&$form, &$form_state, $form_id, $element) {
   if ( TRUE === in_array( $form_id, array( 'user_login', 'user_login_block') ) ) {
       $form['name']['#attributes']['class'] = array('mgnB10', 'fntXL', 'drkFg1');
       $form['pass']['#attributes']['class'] = array('fntXL', 'drkFg1');
   }
}

I want to include these fields inside a single '' and assign class name for design purpose. For example:

div class="class-name"
    username field
    password field
/div

I gave the following code but I didn't get expected output:

function mymodule_form_alter(&$form, &$form_state, $form_id, $element) {
   if ( TRUE === in_array( $form_id, array( 'user_login', 'user_login_block') ) ) {
       $form['user_login']['#attributes']['class'] = array('#prefix' => 'myclass');
       $form['name']['#attributes']['class'] = array('mgnB10', 'fntXL', 'drkFg1');
       $form['pass']['#attributes']['class'] = array('fntXL', 'drkFg1');
       $form['user_login'] = array('#suffix' => '');
   }
}

I have assigned class names to both user name & password text fields in user login block separately like this and it's working.

function mymodule_form_alter(&$form, &$form_state, $form_id, $element) {
   if ( TRUE === in_array( $form_id, array( 'user_login', 'user_login_block') ) ) {
       $form['name']['#attributes']['class'] = array('mgnB10', 'fntXL', 'drkFg1');
       $form['pass']['#attributes']['class'] = array('fntXL', 'drkFg1');
   }
}

I want to include these fields inside a single '' and assign class name for design purpose, like this.

<div class="class-name">
  username field
  password field
</div>

I am using the following code but I didn't get the expected output.

function mymodule_form_alter(&$form, &$form_state, $form_id, $element) {
   if ( TRUE === in_array( $form_id, array( 'user_login', 'user_login_block') ) ) {
       $form['user_login']['#attributes']['class'] = array('#prefix' => 'myclass');
       $form['name']['#attributes']['class'] = array('mgnB10', 'fntXL', 'drkFg1');
       $form['pass']['#attributes']['class'] = array('fntXL', 'drkFg1');
       $form['user_login'] = array('#suffix' => '</div>');
   }
}