I think there is an important aspect of it which Andreas glimpsed over - do it incrementally. While Ladybird is not yet on par with more mature engines it already works for a lot of webpages just fine.
I mean I don't see highly advanced / complicated features like webgl used in my day to day browsing, it's tech demos and the like linked from HN at best. A new browser can do without that for a long time.
