Bulma?

Bulma is a free framework that provides ready-to-use frontend components that you can easily combine to build responsive web interfaces.

...Open-source?

The Bulma framework is a free open-source CSS solution based on the Flexbox layout, which means that Bulma’s original source code is freely available for download — there’s no limit to how far you (and the growing Bulma community) can extend its functionality.

...Flexbox?

Flexbox is a layout technique for aligning and distributing space for items in a container where the size of items is unknown or dynamic.

Columns

The power of Flexbox in a simple interface

Form

The indispenable form controls, designed for maximum clarity

Components

Advanced multi-part components with lots of possibilities

Modifiers

An easy-to-read naming system designed for humans

Layout

Design the structure of your webpage with these CSS classes

Elements

Essential interface elements that only require a single CSS class