CSS Libraries

Bootstrap

One of the grandaddys on the scene. (There’s an alpha of version 5 as I’m writing this, if I recall correctly one of its headline features is it’s replaced jQuery with just raw modern Javascript.)

Tons of elements, pre-designed components, an icon library, community themes, the list goes on.

Bulma

Very similar in end function to Bootstrap, but with arguably easier-to-use class names. Based explicitly around FlexBox, for naturally responsive pages.

Includes a number of prebuilt components, and no Javascript.

Tailwind

A very different route, Tailwind provides a large number of utility classes that run through a preprocessor. This gives you the full power of writing ad-hoc styles on individual elements, but by making classes for them you can be certain text-purple-500 is going to be consistent where used.

There’s definitely some drawbacks as regards consistency between larger components, but modern HTML (or Javascript for that matter) usually handles that abstraction just fine.

There are a few community-sourced component libraries available, but Tailwind UI by the Tailwind authors is growing constantly more robust.