Does anybody could explain why on my client’s laptop, her portfolio looks like that :

(https://i.sstatic.net/L8ZVa4dr.jpg)

But on every other laptop or cellphone (I’ve tried on another mac with Firefox), it works perfectly and looks like that :

(https://i.sstatic.net/nFutDsPN.jpg)

The code is HTML/CSS/JS with node.js and react. Could it just be a compatibility issue with an old version of her macOS or firefox ?