A fun way to brush up on your Flexbox properties. The Flexbox Froggy game from Thomas H. Park.
Nice article demonstrating how to control Flexbox grids, including how to control gutter spacing.
Some use cases for CSS Calc, a useful CSS function with great browser support.
Some of the newer CSS length units can make creating responsive layouts much easier. Here is a guide to these lesser-known units include vh, vw, vmin and vmax.
24 Ways kicks off for 2015 with an article on Animating Your Brand. I like the approach of incuding animation in the style guide and making sure that animation isn't something we "tack on at the end" of the design process.
Some guidelines for those website background videos that have become popular.