Bootstrap

Bootstrap is a responsive framework for developing with HTML, CSS, and JS. It has many in-builds components, which you can easily drag and drop to assemble responsive web pages.

Features:
1. Bootstrap enables utilization of ready- made blocks of code
2. It ensures consistency irrespective of who's working on the project
3. It offers extensive list of components
4. Base Styling for most HTML Elements
5. Bootstrap can be customized according to the specific need of the project

More Information and Official Website:
Download : http://getbootstrap.com/docs/4.0/getting-started/download/