

Just to clarify for OP:
The format is always SVG, as it’s an open standard. Inkscape is the leading FOSS SVG vector editor. Boxy is a web-based SVG editor which is freemium.
SVG is the W3C standard for vector graphics that can be rendered by practically every browser. Gzip compressed SVG files (svgz) are much smaller but enjoy less support.
Illustrator can import, edit, and export SVG files.
Calibre is one of the great pieces of FOSS software, and demonstrates everything good about FOSS: it has regular updates; it’s been around for simply ages; it works really, really well; it gets updates and new features and yet has never in my memory had a breaking, non-backwards-compatible release… it’s stable; and it resists - in its way - the attempt by publishers to steal our rights and ownerships of our media.
I
contributedonate to Calibre. I hope that Goyal has a successor lined up to take the helm who can continue such an outstanding contribution when he finally retires from the project.Edit: clarification