What's the best framework for creating Multi platform desktop app using web technologies such as HTML, JavaScript, and CSS, I would like to know the complexity (learning rate) and flexibility of each one as much as the future promise of each one ?
Yes, it could be Electron.
f you can build a website, you can build a desktop app. Electron is a framework for creating native applications with web technologies like JavaScript, HTML, and CSS. It takes care of the hard parts so you can focus on the core of your application.
In fact, there is a lot of applications made using web technologies (HTML/JS):
You can take a look at https://electron.atom.io/