What are some alternatives?
When comparing Ninja UI and Semantic UI, you can also consider the following products
jKit - A very easy to use, cross platform, jQuery based UI toolkit, that's still small in size, has the features you need, and doesn't get in your way of doing things!
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
JQWidgets - Unique toolkit for HTML5 Web development
Foundation - The most advanced responsive front-end framework in the world
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces