Base — CSS reset/normalize plus HTML element styling.

Base rules consist of styling for HTML elements only, such as used in a CSS 
reset or Normalize.css. Base rules should never include class selectors.

To avoid 'undoing' styles in components, base styles should reflect the
simplest possible appearance of each element. For example, the simplest
usage of the ul element may be completely unstyled, removing list markers
and indents and relying on a component class for other applications.

https://drupal.org/docs/develop/standards/css/css-file-organization-for-drupal-8
