JavaScript itself (ES language changes), APIs callable from JavaScript (there are a lot), CSS, HTTP(s)/TLS and evolving security, ever-evolving compatibility requirements to keep popular pages working, performance expectations of the rendering model (what must be smooth nowadays didn't used to be and it's expected by sites), things like WASM, WebGL (new version on its) & WebGPU, the sandbox, evolving security patterns (HTML/HTTP/JS security model is complex) and dealing with new security issues (it never stops), compatibility with OS version releases (always new issues), compatibility with GPU drivers (always new issues), video codecs.
These things by themselves require a well-funded team to keep the browser relevant to modern sites. Well-funded because they take a lot of time and ideally to be done to a professional standard. The number of spare-time or independently-wealthy volunteers around to do the work seems thoroughly insufficent compared with what it would take to keep up.
That list doesn't even have new user-visible features (so nothing to "sell" to users except "we still work with current sites").
They are just moving-target "basics", minimal expectations by users and sites to remain "current" on the web. I'm sure I've left off a lot too, it's just off the top of my head.
These things by themselves require a well-funded team to keep the browser relevant to modern sites. Well-funded because they take a lot of time and ideally to be done to a professional standard. The number of spare-time or independently-wealthy volunteers around to do the work seems thoroughly insufficent compared with what it would take to keep up.
That list doesn't even have new user-visible features (so nothing to "sell" to users except "we still work with current sites").
They are just moving-target "basics", minimal expectations by users and sites to remain "current" on the web. I'm sure I've left off a lot too, it's just off the top of my head.