Responsive Qr Code

your solution looks good, but i have some changes that you can make for the next solution. first of all try to use semantic HTML the more possible, so you can use <main></main> for your container.

Then to center correctly an element on the page you can use this css code on the body:

body{
min-height:100vh;
display:flex;
justify-content:center;
align-items:center;
}