From the terms of service: “You agree not to reproduce, duplicate, copy, sell, resell or exploit any portion of the Service, use of the Service, or access to the Service without SVGViewer's express written permission.” (https://www.svgviewer.dev/terms)

Apart from that, they let you store and share SVGs on their servers, so it’s not purely client-side.

Even if it were a plain HTML page, or a simple, single-file Javascript web app, that could either be hosted on a remote server that you don't control (as this is), or can be downloaded and hosted on your own computer or server allowing you to modify it.

And although that sort of thing would mean you can just download it straight from where it's being hosted from a technical point of view (for things where there isn't any backend or hidden logic in code at all) there's still the legal question as shown in logic8's comment.