UIKit

UIKit is a really handy web development framework which can really shave of a rather large amount of the development work you will need to do yourself.
I like to use this as an alternative to bootstrap.

UIKit is licensed under the beloved MIT License so use it how you like, just don't try and repackage it for sale.

UIKit comes with many great features like grid layouts, navbars, pagination, modals, scrollspy and a large amount of other handy components.
Much like bootstrap, UIkit comes with the latest browser support so you know that it will work on all latest versions of browsers and devices.

As with most frameworks you can choose the features you want and download a stripped down version of UIkit so your not clogging up your websites with code that will never be used.

If your a web developer I highly recommend having a play around with UIkit :)