QR Code Component solution using HTML and CSS

Hi everyone! I am not convinced of my solution!

1. In order to be able to center the .container div both horizontally and vertically using Flexbox, I set min-height: 100vh on the body. It was a shot in the dark which seems to be working. Could someone remind me of the role of height in relation to flexbox? I've not fully grasped it.

2. I gave that same .container div margin: 100px auto as I also wanted free space at the top and bottom. Is that correct or is there another way?

3. I didn't set a width on the body as it didn't seem to make much difference. Should I have done?

Many thanks!