ptb/flexgrid is a next-generation web page layout framework based on flexbox: the CSS Flexible Box Layout Module. It uses the same responsive 12 column grid and CSS class names as Bootstrap.
A customized version of Modernizr is used to detect support for flexbox. The flexbox layout is supported on Chrome, all versions of iOS, Safari 3+, IE 10+, Opera 12.10+, and Firefox 22+. Other browsers get the fluid Bootstrap 3 layout, except Internet Explorer 6 and 7 which use a fixed layout.
Unlike other frameworks, it is just a grid, and thus requires only 1594 bytes for flexgrid.css and 2068 bytes for modernizr.js; with gzip both together are less than 1.67k! A display vs. source re-ordering variant is available as an option.
Documentation, a demo, and source code are available. Suggestions for improvement are welcome!