Stencil builds Web Components that run natively or near-natively in all widely used browsers.
Web Components are a set of standardized browser APIs centered around the Custom Elements v1 spec, a cross-browser way to define and create essentially new HTML tags, and is the successor to the now-defunct v0 spec.
Custom Elements are natively supported in Chrome and Safari (including iOS!) and are coming to both Edge and Firefox. In fact, Firefox has native support behind a flag, indicating full native support is near.
For browsers without native support, a small polyfill helps developers use Custom Elements seamlessly and with little performance overhead.
Stencil uses a dynamic loader to load the custom elements polyfill only on browsers that need it. With this polyfill Stencil's browser support is Chrome (and all chrome based browsers), Safari, Firefox, Edge, and IE11.
Web Components are being used in production in 2017 with the above approach.