Zen is a powerful, yet simple, HTML5 starter theme with a responsive, mobile-first grid design. If you are building your own standards-compliant theme, Zen is an excellent starting point. The theme includes extensive online documentation and comments in its PHP, HTML, CSS and Sass files.
Zen has been the most popular Drupal theme framework for many years, but is now starting to get real competition from Bootstrap. Either way, both ar excellent choices.