C++ is at least an order of magnitude more complex, but there have been plenty of C implementations, some even entirely the work of a single person[1]. "I wrote my own C (subset) compiler" seems to be a reasonably common thing on HN too.[2][3][4][5][6][7]
On the other hand, I haven't see very many "truly independent" webpage rendering engine implementations (e.g. HTML4 or HTML5-subset, CSS2.x), so if anyone wants to give it a go (or Go, if you like...), they are more than welcome to, if only to increase the diversity of available implementations --- something that could probably handle HTML4/CSS2 might not be all that difficult, and especially so if you don't care for 100% identical results to the mainstream browsers (which often differ slightly too.)
[1] https://en.wikipedia.org/wiki/Tiny_C_Compiler
[2] https://news.ycombinator.com/item?id=13914137
[3] https://news.ycombinator.com/item?id=15463738
[4] https://news.ycombinator.com/item?id=13778353
[5] https://news.ycombinator.com/item?id=8558822
As someone who did a lot of the work to implement a from-scratch HTML4/CSS2 engine, I struggle to come up with words that would adequately express to you just how much you are underestimating the difficulty of this.
If anything, the monstrosities produced by committees are less powerful and less beautiful. (That we put up with them says more about us.)
Yes, it is huge and gigantic, but worth it's weight in gold. The stuff "just works" and has almost no limits. I'd say that spec is a masterpiece.
Also, do we count all the pages expended on SRFI's? Or not?
So yeah the web is complex but so are most popular runtimes.