QR Code Component using basic HTML and CSS

Q1 I know that media queries are used to make a page responsive to variations in viewport size but how do I implement this, say for this case I wanted the qr code to occupy the full height of the viewport whether it be using a desktop or mobile screen. How do I "resize" the html elements accordingly?

Q2 What are the problems associated with the code and any best practices that could be recommended to facilitate a more efficient code and way of coding?