I think the HTML, CSS and even JavaScript are the most stable and future-proof components of your stack. Your Rails backend, on the other hand, will experience far more changes and API instability in the long run.

JavaScript was considered as a unstable and under-specified part of the Web in the "Dynamic HTML" era somewhere between 1997-2006, when Microsoft Internet Explorer implementation of DOM diverged from more standard Netscape/Firefox in many tricky ways. This has largely been solved by better standards, initiatives like Acid tests and (unfortunately) slowly spiraling into Blink engine monoculture.

> Your Rails backend, on the other hand, will experience far more changes and API instability in the long run

I see this as different layers of stability. On the bottom is a solid foundation of web standards that's widely adopted and resilient to breaking changes. Layered on that is the web framework and language. Before ruby, I really enjoyed perl, so I've experienced the collapse of a language and community. Matz had some good insights drawn from the history of other languages https://www.youtube.com/watch?v=4MM5b2F9zrM

Javascript is fascinating because of its evolution. Its initial popularity and wide install base guaranteed a lot of resources for it to grow. I'm excited to see whether wasm can bring more languages to the web.