Well, it is amazing you can write something in a decent set of Javascript these days and have it execute mostly the same across Windows, Mac, Linux, iOS, and Android, irrespective of CPU architecture.
It is, but at the same time I know that if I re-run it in two years it will suffer from bitrot and ill have to futz around with it to get to run. I hate that a lot.
Surely by "re-run" you mean "try to re-build". If you write something that runs in the browser today (and you're not doing something dumb like using experimental or otherwise non-standard APIs), chances are extremely high that it will still work in two years. The browser is one of the most consistent, interoperable, stable software platforms that the industry has managed to produce.