What are some alternatives?
When comparing Gem.js and Svelte, you can also consider the following products
Vue.js - Reactive Components for Modern Web Interfaces
Preact.js - Preact is a fast 3kB alternative to React with the same modern API. Components & Virtual DOM.
AngularJS - AngularJS lets you extend HTML vocabulary for your application. The resulting environment is extraordinarily expressive, readable, and quick to develop.
Polymer - Polymer is a library that uses the latest web technologies to let you create custom HTML elements.