The World Wide Web Consortium is a collection of member organizations and staff, founded and led by the man who literally invented the web, Tim Berners-Lee. They work to develop web standards, and ensure that the web is an open platform that works for everyone on every device.
The system is a dramatic visual improvement over their old design, with more readable typography and a pleasing color palette. There is, of course, a strong focus on accessibility, and in particular, showcasing how accessibility doesn’t have to be boring or ugly.
There are lots of smart Sass variables (if that’s your thing), well structured markup, and absolutely fantastic documentation.