It's worth mentioning that the flexbox standard has gone through three major revisions, each with very different syntax. To extend the usefulness somewhat to older browsers it's possible to mix the different versions.
Also worth mentioning that you can avoid having to deal with most of the problems of the multiple syntaxes if you're using sass by using bourbon's mixins - http://bourbon.io/docs/#flexbox (or you can just not care about < IE11...)
See this article at CSS-Tricks for some examples https://css-tricks.com/using-flexbox/