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.