Mature how? It seems like it's already more or less what they're going for.
I wasn't thinking of a toy HTML rendering engine to play around with but a browser - an Electron project could pretty quickly exceed what it can do.
Rendering engines are cool though - and it's nice one of has been invited to TC39: hover over linusg on https://github.com/SerenityOS/serenity/graphs/contributors
I'm pretty interested in Servo and hope its reboot after exiting Mozilla goes well. https://servo.org/blog/2023/01/16/servo-2023/
[0] https://awesomekling.github.io/Ladybird-a-new-cross-platform...