I honestly don't know if my flexbox or grid going to work on other browsers (esp the mobile ones!) I saw people wrote different lines just for firefox and opera, but it was from two or three years ago? should I test it myself, installing different browsers, or?

Backwards Compatibility of Flexbox - CSS: Cascading ...

huh. okay, the versions are all streamlined to display: flex; in 2015.

okay, no need to write 4 different lines for each one bit of flexbox property, right? right.