Answer by Ouroborus for Taking vector screenshot from Firefox

gtk-vector-screenshot isn't going to work as you expect from all apps. Some apps use a canvas and do custom drawing to that. All you would get is a canvas placeholder (as you did). gtk-vector-screenshot gets its vector draw commands from GTK and converts them into the equivalent SVG commands. If the software isn't using GTK draw functions to do its rendering, you won't get anything. If you're interested, source code is available here.