Skeleton vs Materialize

No bells and whistles for Skeleton, it's just CSS.

Since Materialize follows Google's guidelines for Material design, which in theory is device agnostic, Materialize itself is device agnostic too. It's designed to look good on every device.

CSS components: Badges, buttons, cards, collections, footer, forms, icons, navbar, pagination, preloader.

JavaScript components: Collapsible, Dialogs, Dropdown, Media, Modals, Parallax, Pushpin, ScrollFire, Scrollspy, SideNav, Tabs, Transitions, Waves.

Mobile-specific: slide-out drawer menu, toasts.

While the default style is not bad at all, Materialize also gives developers the ability to customize it and fit their own style, while still keeping in line with the Material Design philosophy.

Along with the CSS files, designers can also download the SASS files which can be edited and compiled.

Material design is very opinionated on how design elements should behave and look. The basics of which revolve around certain visual elements (physics, space, momentum and light) which are used to create specific UX elements.

This is very helpful because it creates a consistent feel without making every design look the same. This can be seen in Materialize too, where each element may be customized but still it keeps the consistent look of the material design.