What an SVG viewer does
SVG is XML. Browsers already know how to draw it, but opening a raw file in an editor does not show size, viewBox, or transparency. An SVG viewer puts the markup next to a live preview so you can confirm icons, logos, and diagrams before you commit them.
This page parses the text with DOMParser, draws it in a sandboxed iframe, and lists width, height, viewBox, and how many elements sit under the root.
viewBox vs width and height
viewBox is the internal coordinate system. width and height are how large the graphic appears on the page. Keep them in the same ratio unless you want stretching. If the preview looks cropped, the paths probably sit outside the viewBox.
Data URIs and QR SVG
A data URI inlines the graphic. Handy for a one-off CSS background. Bad for a 200 KB illustration you reuse on every page.
If you exported SVG from the QR Code Generator, paste it here to check quiet zone and contrast on light or dark. Messy HTML around an inline SVG can go through the HTML Beautifier first.