Ladybird was born from SerenityOS, which is a hobbyist unix-like (or POSIX compliant?) OS that simply aimed to do things "from the ground up". It just happened that they needed to make a browser, and the response was to make one from scratch.
From there it seemed to have brought a lot of attention organically to the point where it can stand on its own, but originally it was never intended to be a "third browser engine" from its inception.
Sure, but an individual website may use only a few of those standards. Ladybird devs will pick a website they like to use - Reddit, Twitter, Twinings tea, etc. and improve adherence to X or Y standards to make that one website look better. In turn, thousands of websites suddenly work perfectly, and many others work better than before.
Ladybird is largely conformant to the majority of HTML standards now. It's about the edge cases (and where standards aren't followed by websites) and performance. This isn't a new project.