QR code using HTML and CSS (flexbox)

What are you most proud of, and what would you do differently next time?

It seems like my result is very similar to the preview and I'm proud of it. I know that I use "old" techniques (for example px instead of rem/em ect.) But next time I will try to use them.

What challenges did you encounter, and how did you overcome them?

My problem was that the QR code was not in the center. I mean.. It was at the center in a main cross but not in the center of the webpage. It was too high. I used a margin but it was not good solution. But then I relized that I can use min-height: 100vh; and min-width: 100vh; and that solved my problem. I hope it is the "right" way.

What specific areas of your project would you like help with?

I'm not sure about sizes. How do I figure out if I should use 300px or 350px? In this case.. I guesstimated it. Maybe I'm little bit dumb and there is a chance that I overlooked it in instructions. I'm not sure about it.