Jeste li počeli koristiti flexbox za domaće sajtove? Ako da, javljate li korisnicima starih browsera na neki način poruku da si upgradeaju?
Flexbox ne radi na IE 10 i manje, Safari 9 i sl.
Samo IE11+ podržavamo, ostali nisu vrijedni spomena 
Bootstrap 4 je isto raden u sa flexboxom tako da i on podrzava samo IE 10+ 
Internet Explorer
Internet Explorer 10+ is supported; IE9 and down is not. Please be aware that some CSS3 properties and HTML5 elements are not fully supported in IE10, or require prefixed properties for full functionality. Visit Can I use… for details on browser support of CSS3 and HTML5 features.
If you require IE8-9 support, use Bootstrap 3. It’s the most stable version of our code and is still supported by our team for critical bugfixes and documentation changes. However, no new features will be added to it.
Koristim
Evo tu sam ga koristio, jednostavno najbolja stvar koja se desila CSS-u u zadnje vrijeme.