HTML, CSS and Javascript. Most of these electron apps are basically wrappers around actual websites to give a place in the dock and show notifications and access the filesystem.
But that isn't what's missing. It's a restatement of the problem. DOM based apps are much more resource intensive than native. What is missing from native that makes business choose DOM?
If there was some modern tool like WxWidgets that supported modern apis like DOM, Android and UWP, would we see more use of native? Electron would therefore become pointless.
The hypothetical business has two choices. Choose Electron, or choose some other toolkit that has native, cross-platform support (like Qt). It's far easier for the business, and the developers there, to take their existing website HTML, CSS, and Javascript; and simply wrap it in Electron (which costs $0), and call it a day. Every other choice is (perceived as being) more expensive.
Qt is a modern toolkit with native-cross platform support, but costs money for commercial use, and businesses and software developers don't want to spend the money on it.
