What are some alternatives?
When comparing lhorie.github.io mithril and Polymer, you can also consider the following products
Vue.js - Reactive Components for Modern Web Interfaces
AngularJS - AngularJS lets you extend HTML vocabulary for your application. The resulting environment is extraordinarily expressive, readable, and quick to develop.
Aurelia - Next gen JS framework written with ES6 and ES7. Integrates with Web Components. No external dependencies except polyfills.