So, what is flexbox?
It's a module that has been designed to provide a more efficient way to layout and distribute space around items in a container, particularly if their sizes are not yet known. We can set a number of properties to ensure that each item best uses the available space around it, even if its size changes.
At the time of writing, this is a W3C Candidate Recommendation; this means that it is effectively on the last call before becoming a browser standard in late 2016. This should be something of a formality though, as most browsers already support it as a standard: