See it in action at https://interactivetext.github.io/. To get started:

1. install emscripten, the compiler that does most of the work
2. download this repository
3. open a console in the repository folder (if on Windows, shift right-click when nothing is selected), and compile with this command: emcc o.cpp sample.cpp --shell-file shell.html -o index.html -std=c++1z -O3 --memory-init-file 0 --pre-js pre.js -s WASM=0
4. run index.html to test the compiled files. Internet Explorer won't work for this step; it places restrictions on local files.
5. to start writing, modify sample.cpp, and compile again to see your changes.