Bootstrap is still the best HTML + CSS framework out there to build quality websites with ease. They are free, open source and very powerful library of component which developers around the world rely on
If you don't know, you can build a website by simply copying and pasting certain snippets of code into your text editor. That, of course if all the dependencies are met. You can view the components before you use the code. It is pretty straightforward and easy to use framework.
A lot of framework came along the same line. But I am not sure anything grew to the popularity of Twitter Bootstrap. They are still on Github and widely supported by a huge community of volunteers.
Good luck Bootstrapping