Responsive QR code page

It has nothing to do with using rem. As you're supposed to be using them when you want to make things responsive. This seams to me to be the culprit: width: 17svw; on your #qr. Which on smaller screens you give a 75vw. This means on tablets and smartphones, the qr will take up about 75% of the screen width. But on desktops, it will shrink down to 17%. What you need to do here is, give your div id='qr' a max-width. There is various ways you could write this, but an example could be:

* {
	box-sizing: border-box;
	margin: 0;
	padding: 0;
}

#qr {
	width: 95vw;
	max-width: 350px;
	margin: 0 auto;
}

Here we tell the div you need to be 95% of the viewport, until your maximum 350px wide.

Are you familiar yet with how the box-model works in css? CSS Box Model And what the purpose is of this code I added:

* {
	box-sizing: border-box;
	margin: 0;
	padding: 0;
}

Also, don't use div to write headings or text. There are html-tags especially for text. HTML Text Formatting