Of course you are correct that supporting Twitter or any service is a moving target, so long as it changes. But that doesn't mean specific bugs can't be captured in a test case.
At one point, he deletes half the HTML file to isolate where in the site the problematic code is. In a way doing a kind of binary search. After a few iterations of this, he comes up with a very small case that exhibits the problem he's trying to solve.
It's clear he knows his way around the codebase and where to make changes, but isolating these test cases is probably as important. And presumably if you fixed enough of these issues (while following the specs), 99% of the modern web should work just fine.
Watch this video of Andreas doing exactly this, albeit for a simpler web app https://www.youtube.com/watch?v=W4SxKWwFhA0
At one point, he deletes half the HTML file to isolate where in the site the problematic code is. In a way doing a kind of binary search. After a few iterations of this, he comes up with a very small case that exhibits the problem he's trying to solve.
It's clear he knows his way around the codebase and where to make changes, but isolating these test cases is probably as important. And presumably if you fixed enough of these issues (while following the specs), 99% of the modern web should work just fine.