QR Code Component (Responsive )

This Challenge was pretty easy to do. Still making it responsive took me a while as I didn't use any media queries for different screens & yet it's responsive on even the smallest screen size i.e. 240*320 (for keypad phones).

Questions :

• Is there a way to vertically center a <div> perfectly for every screen size without using the display:flex & align-items: center; & destroying the responsiveness of the page ?

• What things should be taken care of while making responsive WebPages ?