Assuming you don't use Javascript, kind of.

Dillo supports XEmbed, so you can run it embedded in another X windows. This is done by Claws Mail to render emails as HTML [1].

[1]: https://www.claws-mail.org/plugin.php?plugin=dillo

There are better and lightweight alternatives for Electron: https://news.ycombinator.com/item?id=38687662

It would, but it's unlikely that the browser implements enough of Javascript or CSS to replace the engine for Electron and to be useful