No, I understand what Andreas is saying. But the reality is, when you read and implement specs for a specific website, you end up cutting corners, even accidentally. Maybe Twitter relies on a particular behavior of fetch() that was screwed up in Chrome 97 and has had to be kept for backwards compat this entire time. Maybe it uses some CSS that never got properly documented or specified.

By targeting a single website, you end up accidentally writing in those site-specific fixes _in_ your implementation. You only realize it's fucked up because you visited Twitter. but maybe it screws up another site. Maybe something else depends on a quarter of that functionality, and you've accidentally broken it.

Maybe so. Hopefully you’d find that out while looking at the spec for that particular function. If not, you may have to rewrite some code as you learn more. Life goes on

It seems that GP already addressed your concern: "read through the relevant specs, and try our best to implement the required features cleanly and correctly"

Of course some errors will be made along the way. That's to be expected, regardless of the approach taken.