"Oh, is this metric important? Let me get right on that."
No shade intended towards the Ladybird team. You were given the terms and you're behaving rationally in response to them. More power to you. It's just a fantastic demonstration of what it looks like to very suddenly be developing against a very specific metric.
Also, I don't think that the Ladybird folks are just doing the bare minimum to only increase their score on WPT. They're implementing each feature in such a way that basic browsing seems to work better and that their WPT score improves.
However, a jump like that means precisely and exactly what I said it means; very suddenly, that metric became much more important to the team. It is written straight into the graph.
A large number of encoding-related tests that were probably relatively easy to fix in bulk is certainly a plausible explanation.
A lot of people are imputing to me assumptions that they are bringing to my post, such as assuming that such improvements must be fake or bad or somehow otherwise cheating. Nope. Moreover, if you are thinking that, don't take it up with me, go take it up with the graph directly. It's not my graph. I'm just amused at the spectacular demonstration of Goodhart's Law.
Are the commentators who think I'm being critical of the Ladybird project going to defend their implicit proposition that the browser got twice as good in whatever period that graph is in, a week or a month or whatever? Of course that's not the case.
Too much useful insight is withheld or misappropriated these days.
Not really, though. The latest jump was from implementing some CSS Typed OM features, which has been in-progress work for a while now. The 6k increase in the test score was a bit of a happy surprise. It's also not that much of a jump when you zoom out and see it's "just" a continuation of a steady increase in score over a long period.
And, in any case, implementing more of the standards is just simply good, and would need to be done at some point anyway.