Even has an irc client ;) everything you need
This is an excellent observation, and gives me an excuse to recommend Alan Kay's seminal OOPSLA 1997 keynote "The computer revolution hasnt happened yet" -- the link to the whole thing is below, but I've set the time at the start of what I think is the immediately relevant point (he takes a minute or so to explain)
There’s almost no commercial insentive to create a browser/rendering engine (Opera couldn’t make it work), and it almost to much work for an open source project to take on. As sad as it might be, Google won and Chromium will be the final browser, everything else is just customization. At least Microsoft had the courtesy of slowing down progress with Internet Explorer, allowing others to more easily catch up.
Of course the difference between building an OS and a browser is that for an OS, you just have to build something that is usable, how you get there and what that looks like is up to you, and you can really get creative with it and break norms.
For a browser engine, by definition, you have to build something that behaves basically exactly the same as any other modern browser. So you're implementing a gargantuan spec to a very high level of precision (since you have no control over how it's used), and the end result is a technical feat that's only impressive in the fact that it was done, not in that you get to use it now, and it does anything a reskinned chrome wouldn't do.
There are hundreds of standards spanning thens of thousands of pages. Some of them are obsolete, some of them are not, most of them interact with each other in complex and non-obvious ways.
An attempt to measure the scope is here: https://drewdevault.com/2020/03/18/Reckless-limitless-scope....
And that's before we get to things like Javascript or WASM.
Another measure is the count of WebAPIs. A modern browser ships six to seven thousand APIs: https://web-confluence.appspot.com/#!/confluence All these APIs are available via Javascript
The methodology used there is horrendously bad. Drew's smart, so it's hard to conclude that this isn't intentional just so he can pump up the numbers and tell cute stories like the one about Wikipedia's list of longest novels. (Not to mention, an overabundance of specification of correct behavior isn't what makes implementation hard, it's trying to match the undefined behavior that everyone else follows without having a spec that makes things hard.) Any serious attempt to measure the scope of what browsers actually implement is pretty straightforward, so there's no real excuse: you start with the the HTML5 spec and then go from there.
Not sure on the web browser though