Bootstrap is perhaps the most widely used front-end framework on the web. It was released in August, 2011, and has been under continuous development ever since.
Bootstrap 4 was a long time coming after Bootstrap 3, but was worth the wait. It is accessible, and mobile. Implemented correctly, it complies with WCAG 2.0 (A/AA/AAA), and Section 508. AAA compliance is an impressive achievement. Kudos to the Bootstrap team.
Skilling comes with a custom theme based on Bootstrap 4. Although the theme could be used for any Drupal site, it is designed for students taking skills courses. For example, students doing tasks will want to split their screen space. One half will have a browser showing a Skilling course. The other half of the screen will have and a worksheet, code editor, or whatever students use.