This is a little framework (just a collection of LESS/SCSS/Stylus mixins) by Tyler Tate for creating responsive grid-based CSS layouts: you declare the layout with variables in a CSS pre-processor, and then it calculates the actual pixel widths. It is what I've used for my few designs, but I might try out CSS flexbox in the future...

Here's the project's github (there are several pending PRs, and Tyler hasn't made any merges since July 2015): https://github.com/tylertate/semantic.gs