It is very good if we can create a pure CSS multilevel drop-down menu in Drupal. We can simply avoid adding additional modules or jQuery for the multilevel drop-down menus. As we had a requirement to create a multilevel drop-down menu without using jQuery, we created it in pure css. And it works well in different browsers.
We created this menu as main menu for the header navigation part. And there is a region for the header part called "region-header" and it wrapped in a class called .header-wrapper. Also Drupal provides a class called 'menu' for ul. So we had to define the class-name along with the ul.