Pure CSS
Only 3.8KB minified and gzipped, but packed with features! Pure CSS has been my lightweight framework of choice for years now, including when working with an outsourcing software development team.
Info & download: https://purecss.io/
Bulma
With 21kB minified and gzipped, Bulma isn’t the most lightweight of this list, but this new framework is so interesting that it totally deserves a mention. Bulma is built with mobile-first approach, which makes every element optimized for vertical reading, and its grid is fully built with Flexbox. Achieving flexible layout with same-size columns is as simple as adding .column class to your HTML element.
Info & download: https://bulma.io/
Mini.css
A lightweight (7kb gzipped) framework for crafting a fast site easily.
Info & download: https://chalarangelo.github.io/mini.css/
UIkit
UIkit is a lightweight and modular front-end framework for developing fast and powerful web interfaces.
Info & download: https://getuikit.com/
Skeleton
With only 400 lines of code, Skeleton is light as a feather. Yet, it features a grid, typography, forms, media queries… all you need for building a quality site in no time.
Info & download: http://getskeleton.com/
Base
Base is a very simple but robust css framework. Built with lightweight and minimal code, Base is designed to provide developers and designers an easy way to built cross-browser, mobile-first websites and apps.
Info & download: http://getbase.org/
Spectre
A lightweight (~10KB gzipped) starting point for your projects, Spectre provides elegantly designed and developed elements, as well as a flexbox-based, responsive and mobile-friendly layout.
Info & download: https://picturepan2.github.io/spectre/
Milligram
Milligram provides a minimal setup of styles for a fast and clean starting point. With only 2kb gzipped, this tiny but powerful framework is in the top three of the lightweight framework available.
Info & download: https://milligram.github.io/
Dead Simple Grid
Not really a framework but just a simple grid layout that can be used in every project. 250 bytes of CSS code.
Info & download: https://github.com/mourner/dead-simple-grid
Picnic CSS
With a size of only 3kb when gzipped, Picnic is for sure lightweight, but also has everything you need for creating a great looking, functional website.
Info & download: https://picnicss.com/
Any other lightweight frameworks you like? Leave a comment below.