Andreas Kling wrote a lot of Ladybird himself. It's not finished yet, and it's turned into a big community project with full time employees now, but with some experience building browsers, the modern spec evidently makes it quite a reasonable task to build a web engine. There are a lot of IDL files and whatnot to parse and process documents and the rendering algorithms are almost all laid out in the spec these days.

I tried Servo last month and I must say that after what I've seen Ladybird do, I was kind of disappointed. I don't think release is very close based on the problems I've encountered.