Just getting started on Electron myself. Used by Atom, Slack, Visual Studio... It's worth a look ;)
Electron - Build Cross-Platform Desktop Apps With HTML, JS, CSS
Electron only works for desktop applications and probably needs a js-framework to work good. I think that React + Electron looks like a nice combo and then you can use React Native to create Android and iPhone apps also, probably sharing a lot of code between all versions.
At the moment the pairing seems to be Electron + Cordova for non-React JS frameworks; with those two you can share 100% HTML at least if you have a responsive CSS design.
Also, now that React Native is extending into the Desktop space more directly (with Linux, Mac, and Windows UWP variants), for React you may not need Electron at all.