Normalize.css library was added to core (reset.css replacement)

Previously, many themes used a reset.css to ensure a consistent styling across browsers.

Drupal 8 ships with the normalize.css library, which can be leveraged by all themes.

Drupal 7

mytheme.info:

stylesheets[screen][] = reset.css

Drupal 8

1. Remove the line from your .info.yml file.

2. Add this to your template.php instead:
   

   /**
    * Implements hook_preprocess_HOOK() for html.tpl.php.
    */
   function mytheme_preprocess_html(&$variables) {
     // Add normalize.css as first loaded CSS file to all HTML pages.
     drupal_add_library('system', 'normalize');
   }