QR Code page with just HTML and CSS

There many things that I forgot just by doing this project like how to center a div, It's okay to forgot somethings like that when I still begginer?

I learned a lot with this too, like flexbox that's a bit new for me and box-shadow... I think that my shadow version is not perfect like the default version.

The last question is... Yea I can remember somethings about CSS or HTML but sometimes my mind went blank. My English is not that good but I hope you can understand a bit. :)