[seivan]

Just want to point out that http://flexboxfroggy.com doesn't work properly on Safari Version 8.0.8 (10600.8.9)(Mac OS)

On a side note, is it okay just to know Flexbox, or would a requirement be that you can manage without? Without ready made grid classes as well?

[metasean]

I'd definitely recommend learning the traditional css (i.e. not flexbox) because that is what most existing sites are written in and most front-end developers know, so you're likely to need it.