I think you have the causality incorrect. There aren't fewer implementations because the standard is complicated; the standard is the product of the browser wars, when well-resourced browser implementors differentiated themselves by adding behavior to anemic/underspecified standards. Browser vendors did, to their credit, co-operatively add a lot of common/similar behavior and then work to standardize it--that's healthy standards growth. But a lot of the size of the standard is because they all wanted to add different capabilities to entice users.
A standard that's the product of feature competition between pre-existing nonstandardized behaviors is doomed to be huge. See also: AMQP, C++.
The browser wars are long gone, and there is no technical reason why the standards each browser added had to be complex per-use-case builtins instead of composable primitives.
In any case, that is what happened.
And now new projects like Ladybird take a decade to arrive, even with AI, because of the immense amount of stuff in the platform.
> standard that's the product of feature competition between pre-existing nonstandardized behaviors is doomed to be huge.
None of the Unixes are this bloated.