HTML and CSS are all you need to create a Design System that unifies the visual appearance of your products, but as soon as you want to create consistent behavior between applications, you'll need to turn to JavaScript. More often than not, using a modern framework such as React, Vue, or Angular will save you lots of time, and keep you from reinventing the wheel.
In this talk, I will discuss how moving interactivity into the Design System is not only a good choice today, but is most certainly how you will be creating Design Systems in the future. I'll also be discussing the React-powered Design System I've been helping to create which powers a few apps you might have heard of, including SharePoint, OneDrive, and Outlook for the web.