You could even write your own browser that uses python instead of javascript. However, it would only be compatible with python html pages. We stick with JS for web development because that is how the "browser framework" is designed. HTML, CSS, JS, etc. are all just part of that framework. If you wrote your own browser, you would need to reinvent all that as well.
I do not miss the early browser days, where different browsers could have been seen a different frameworks. Things go allot better when the browser framework standardized. I still suffer from the IE trauma.
Allot of what we see with JS frameworks ought to baked into the browser framework, but that process is uber slow, because people have to a agree, and people do not like to agree. But it is magic when they do.