One of the most powerful features of Web Components is using Shadow DOM & CSS Custom properties to achieve actual code sharing among your style guides & pattern libraries with your website and web application code. No more developing design systems in isolation and struggling to have design updates applied to the actual development code. The code you write in your Web Components is both the living pattern library and the code your components use! In this talk you'll learn the specifics of working with Polymer, the most popular Web Components library, to build your design system in a way that can be used across any web-enabled device. We'll cover best practices for working with CSS in Polymer to ensure design and accessibility needs are met. We'll also demonstrate the best ways to deal with the limitations of the Shadow DOM & CSS Custom Property shims to support older browsers.