Comment by Gigachad

A proprietary format would allow doing anything, without having to worry about how other programs will support it.

There’s a conflict between what is needed to simply display images, and what is needed for an ideal editable document.

A web browser doesn’t need the complexity of non destructive Boolean operations, but an editor program does. It can all be exported to SVG at the end once it doesn’t need to be edited anymore. Things like variable thickness outlines can just be turned in to filled Bézier curve shapes that happen to be the shape of the outline.