The browser is more of a universal user interface than a universal OS.
Of course something like chromeOS/ChromiumOS is an OS what boots directly into a browser, but it’s not a universal interface.
Maybe WebOS was a step in that direction being a mobileOS that was all html and JavaScript.
Screenshots: https://www.webosose.org/docs/guides/getting-started/webos-o...
https://www.webosose.org/docs/tutorials/web-apps/developing-...