Front-End Developers Develop For...
A front-end developer crafts HTML, CSS, and JS that typically runs on the web platform (e.g. a web browser) delivered from one of the following operating systems (aka OSs):
- Android
 - Chromium
 - iOS
 - OS X
 - Ubuntu (or some flavor of Linux)
 - Windows Phone
 - Windows
 
These operating systems typically run on one or more of the following devices:
- Desktop computer
 - Laptop / netbook computer
 - Mobile phone
 - Tablet
 - TV
 - Watch
 - Things (i.e., anything you can imagine, car, refrigerator, lights, thermostat, etc.)
 

Image source: https://www.enterpriseirregulars.com/104084/roundup-internet-things-forecasts-market-estimates-2015/
Generally speaking, front-end technologies can run on the aforementioned operating systems and devices using the following run time web platform scenarios:
- A web browser (examples: Chrome, IE, Safari, Firefox).
 - A headless browser (examples: phantomJS).
 - A WebView/browser tab (think iframe) embedded within a native application as a runtime with bridge to native APIs. WebView applications typically contain a UI constructed from web technologies. (i.e., HTML, CSS, and JS). (examples: Apache Cordova, NW.js, Electron)
 - A native application built from web tech that is interpreted at runtime with a bridge to native APIs. The UI will make use of native UI parts (e.g., iOS native controls) not web technologies. (examples: NativeScript, React Native)