Not nearly as mature as the examples you gave, but you might like Ladybird [0], the SerenityOS community has a really welcoming vibe imo.
[0] https://awesomekling.github.io/Ladybird-a-new-cross-platform...
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/
Just kidding but none of the major browsers feel like proper open source projects. Mozilla hasn't had a community-driven vibe in a long time, and its Temporary Extension nonsense make it feel less open in a different sense besides the code. Brave is a grift. Safari is nice but closed-source. Chrome is my favorite but is too tied to Google. Edge started off good but Microsoft has added junk to it.