Microsoft has released a new framework for developing an awesome front-end called Office UI Fabric. So the big question is why did Microsoft created this framework? We already have a ton of different frameworks for front-end development like Bootstrap, Semantic UI, 960 Grid System and many more.
The short answer is that we did not have a framework for creating Office or SharePoint add-ins that integrates with the looks of Office applications.
Since the introduction of SharePoint 2013 and Office 2013 we as developers have a new option to integrate our applications in SharePoint and Office. The add-in model is in basics just a web application that integrates in SharePoint and Office. Until a few months ago we could not make our add-in look like a part of Office. Andrew Connell wrote a great blog in February of 2015 that we needed guidelines for integrating in Office. The answer to this is the Office UI Fabric that is posted on GitHub.