Front-end frameworks are incredibly popular among many web developers. They make it immensely easier to create visually and functionally modern websites. Compatibility with the most important web browsers and the usually large selection of UI components are further advantages. But if you fire up the search engine, you'll find one the fastest: Bootstrap. But there are many others that can also be the better choice depending on the project.
Foundation is another framework with a large number of predefined elements. Besides the individual components, there are also completely assembled HTML templates, blocks or UI kits. On board are mobile-first and responsive design, in addition to the ability to customize elements with attributes. However, customization has its limits here. So if you value developing a completely custom theme with many customizable options, you should rather take a look at Semantic UI. For beginners with little programming experience or for a project start with fast progress, Foundation should be ideal.
The name says it all: Milligram is a very minimalistic CSS framework that is only two kilobytes in compressed size. It is limited to the most important components in a simple design. If you want to focus on speed and the essentials for your website, you should consider Milligram over the larger competition from Bootstrap and Co.