[janfoeh]

If you're willing to include vendor prefixes and all three syntaxes, you can get considerably better support, including IE10: http://css-tricks.com/using-flexbox/

[grumblestumble]

IE10 uses a substantially different interim spec syntax that is conceptually different in a couple of key ways.

[jonrimmer]

If you run your CSS files through Autoprefixer[1], it's possible to use the latest syntax, and have it automatically converted to all necessary older ones.

[1] https://github.com/postcss/autoprefixer