It sounds like your version of Safari may not be recognizing display: flex
.
Older versions of Safari may provide spotty support for flexbox, and full support was not offered until version 10.1 (released March 2017).
Try replacing display: flex;
with the following:
display: -webkit-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
Here's a codepen that you can use to test with your version of Safari.