This is an overview of Drupal 8's adoption of SMACSS for CSS architecture and file organization, along with some of the best practices outlined in the documentation at drupal.org. You can checkout the links at the end for reference.
SMACSS stands for Scalable and Modular Architecture for CSS and is a guide to CSS structure written by Jonathan Snook.
CSS Architecture for Drupal 8
The CSS Architecture for Drupal 8 aims to apply emerging best-practices for CSS, including the SMACSS system of modular CSS architecture.