QR Code with CSS Grid

I would suggest using 'rem' units for most things, since the sizing is based on the root font-size and it will give consistent & predictable results.

However, in certain situations, it could be helpful to have the sizing based on the font-size of a particular element. For instance - if you have set the font-size on a button to something other than 1rem, using 'em' units to set the border-radius will give you better control.

I hope that made sense... if not, or if you want more color, here's a good article

Honestly, your CSS looks great to me, as a bit of a novice, mind you. I did want to answer your question though. To keep it short, em is used for things you want to scale to the font-size of the parent element, and rem is when those elements don't need to care about the parent font-size.

Excellent job by the way!