It's "just" a renderer for a subset of HTML/CSS. Things like this existed for a long time, and even smaller (e.g. https://github.com/libRocket/libRocket).

Electron on the other hand is a full browser runtime, for better or worse. It probably would make more sense to create a cross-platform application packager which embeds the operating system's web-renderer into a window. But I think one of the main selling points of Electron is that you're guaranteed a specific version of the browser runtime, so your app suddenly won't stop working with a browser update.