FreeCodeCamp has a great online curriculum for learning HTmL & CSS and then Javascript and React if you want.

Any thoughts on what would be the best way to develop for Windows, Android, iOS, MacOS, and Linux with as few technologies as possible, especially using HTML-CSS-Javascript? I like Electron(electronjs.org), but Electron doesn't support Android and iOS. I'm thinking Electron for Windows, MacOS, and Linux desktops while using React Native for the mobile devices.

I like what Microsoft is trying to do with UWP apps. It's the only game in town if you want to run the same application on both your desktop and mobile device. The only problem is that there aren't enough Windows 10 Mobile users to drive the demand for UWP apps. If Windows 10 Mobile could get just 20% of the mobile market it would change everything.