When you open sites on other "new" browser engines you typically get a really butchered visual result, with layouts completely broken, elements missing, wrong colors, etc. For example, Servo didn't support floats until recently, and IIRC even simple sites like Hacker News look "wrong".
Ladybird's approach has been to start with a somewhat naive implementation of features, then choose popular websites and apps and just continuously iterate to make them gradually look better, by fixing the parts that stand out. This pragmatic approach means that their supported feature set, while nowhere near 100%, can decently render 90% of websites due to being aligned with the most commonly used features.