I really like sciter for webview-type apps. It's really lightweight, integrates well with native code, and brings some native-app concepts to JS (like the communication between multiple app windows). Sure, it's not open source, but you can have the source code if you pay for it, I can respect that.

My main issue is that it's lacking some of the DOM API. It's complete enough that you wouldn't really notice when writing code yourself, but good luck finding a charting library that runs without modifications.

Sciter is very interesting. It uses a custom rendering engine with a custom fork of QuickJS. Extremely lightweight and fast. Quite a nice project honestly.

It's not open source and there is nothing about Servo from what I could find