6 recommended

This let's you build your own components using functional CSS rather than just using an off the shelf component. It also has a library of components that you can grab.
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2
Sakura: A minimal CSS framework

Sakura is a classless css package that lets you style a page simply by adding in a link to the stylesheet and you should see changes instantly.
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1
Google Material Design

Google's new visual look

Frederik WallerProduct enthusiast studying in the UK. · Written
A lot of great components and already familiar to a lot of users! However, maybe this is not "lightweight" enough for you... Check it out!
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1
Skeleton 2.0

A dead simple, responsive CSS boilerplate.

Skeleton is a lightweight CSS package that has around 400 lines total and built with mobile in mind.
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1
Material Design Lite

Kunal Bhatia22Co-founder & Design Lead @SlidesUp · Written
Material Design Lite may work for your needs. @addyosmani did a great job stripping the library down to the bare minimum.
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Angelos Chalaris made this product
A lot lighter than Bootstrap, yet has a lot of the features you'd expect to find and use in Bootstrap (Grids, Cards, Navigation etc.). Responsive, mobile-optimized and easily customizable, as well as very well-documented, I'd suggest you give it a try!
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