Porting vanilla (minimal external API usage, no SIMD, no threading) C++ code to work in the browser with emscripten and WebAssembly is amazingly simple now. The Emscripten docs are excellent, but here are some things that could speed up the process of getting started:

1. Beware of code which does unaligned memory reads/writes (which "may be slow on some CPU architectures")
2. Test with -fsanitize=undefined -fsanitize=address
3. Link with "-s ALLOW_MEMORY_GROWTH=1" and "-s INITIAL_MEMORY=X", X is a multiple of 64k, allows the C++ heap to grow.

(I completely avoid unaligned memory reads/writes because when compiled to asm.js they "can fail silently".)

4. Link with "-s MALLOC=emmalloc" to reduce compiled size.
5. I usually test with -O1 because -O0 can take very long to load/execute.
6. For debugging link with -s DEMANGLE_SUPPORT=1 and -s ASSERTIONS=1

7. C++ printf() outputs to the Developer Console: Chrome Settings->More tools->Developer tools
8. If something crashes, try running in Firefox which may explain the error differently.
9. I use Web Server for Chrome for