Modules can no longer add stylesheets/scripts via their .info.yml file

Drupal 7 allowed to add stylesheets and JavaScript files via the .info file of a module:

mymodule.info:

stylesheets[all][] = css/mymodule.theme.css
scripts[] = js/mymodule.js

All assets being defined in module .info files were loaded unconditionally on all pages.

Drupal 8

The module .info.yml file properties stylesheets and scripts have been removed.

Module developers should declare their assets as proper libraries in a *.libraries.yml file. More information can be found in #2201089

Attach the library where needed via #attached:

NOTE: The sample below is no longer correct, due to Added hook_page_attachments(_alter)() and removed hook_page_build/alter(). Now use hook_page_attachments() instead.

  $build['#attached']['library'][] = array('mymodule', 'drupal.mymodule');

Use hook_page_build() to attach your library/assets to every page (i.e., resemble the previous effect of the stylesheets[] and scripts[] module .info file properties):

/**
 * Implements hook_page_build().
 */
function mymodule_page_build(&$page) {
  $path = drupal_get_path('module', 'mymodule');

  // Declaring a proper library including its dependencies, and
  // attaching it is the recommended way:
  $page['#attached']['library'][] = array('mymodule', 'drupal.mymodule');

  // Deprecated: Add individual CSS/JS files to every page:
  $page['#attached']['css'][$path . '/theme/foo.css'] = array('every_page' => TRUE);
  $page['#attached']['js'][$path . '/js/foo.js'] = array('every_page' => TRUE);
}