Material-UI is described as 'Our vision is to provide an elegant React implementation of the Material Design guidelines that can be customized to fully match your brand' and is a CSS Framework in the Development category. There are more than 50 alternatives to Material-UI for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Mac, Windows and Linux. The best alternative is Bootstrap, which is both free and Open Source. Other great apps like Material-UI are Materialize (Free, Open Source), UIkit (Free, Open Source), Semantic UI (Free, Open Source) and Foundation (Free, Open Source).
Polymer is a library that uses the latest web technologies to let you create custom HTML elements. Build anything from a button to a complete application as an encapsulated, reusable element that works across desktop and mobile.