React.js gets loads of hype, and likely rightfully so. #1 is that it's just a view library though, so don't let the hype make you think it's the next Backbone.js-style framework if you haven't done your research.
A free 20 video course to learn CSS Flexbox
Google lays down some knowledge about optimizing CSS delivery. Recommendations include inlining small CSS files, not inlining large data URIs, and not inlining CSS attributes (