UIKit! We cannot stress how much we love UIKit.
UIKit is a lightweight and modular front-end framework for developing fast and powerful web interfaces. UIKIt is available at https://github.com/uikit/uikit for free under the open source MIT License. UIKit's open source status is a great thing for the evolution of the internet. This means the web development community at large will help to develop and further this software by donating their time and efforts.
UIKit can be used to style your html and can really give your site an elegant feel. The uikit.css file can be modified to acheive any aesthetic you desire and for the web developer the underlying use is the same regardless of the actual styling of the website being built.