Bootstrap and Foundation both do a good job but they're too heavy.

They're great for prototyping but every time you start a real, front-facing, project with them you have to overwrite lots of designer styles that do nothing to add to the functionality of the website. That's annoying!

Responsive has been built with that in mind. It is a minimalist framework for rapidly creating responsive websites, specifically designed and coded to be as lightweight as possible to prevent the need to undo styles set by the framework itself and allow developers to write efficient code. Most CSS styles are there to simply normalize behaviour across devices or provide hooks for further styling.

Responsive is tiny. The combined CSS and JavaScript is only 15kb minified and gzipped. (44kb minified.), but there is a lot of functionality built into the framework. It's designed to be dropped-in, as-is to your website such as you would with normalize.css.

Browser support covers IE8+ as well as all other modern browsers.