Responsive QR code page using HTML & CSS

What are you most proud of, and what would you do differently next time?

Going back to basics after exclusively working with react/react-native for building UI's over the last few years.

Next time I would take more time creating more media query breakpoints.

What challenges did you encounter, and how did you overcome them?

Remembering the very basics eg. importing a stylesheet into the HTML etc

Overcome with good old googling!

What specific areas of your project would you like help with?

Please recommend me any styling tips.

Any recommended changes to the structure of the stylesheet?

Or any recommendations for improved responsive design handling?

What is best practices when working with different font-sizes? Should I be using rem over px?