I'm always looking for independent from-source browsers that can handle Web APIs (not just standard JS) and perform well. V8, Spidermonkey, and Webkit are the only ones I know of, and that puts some serious limits on choices. Every once and awhile I'll try out some of the browsers from scratch (forget the name) but they still suck, and my code often uses stuff on 2025+ browsers, so I have to stick with those engines. Another option I've used for Webkit is wxPython. But... yeah... Tauri came to mind with the requirements you listed. My bug-out bag browser app is V8 based (just grab a binary zip of Electron and add 20 lines of code and you have a decent browser for offline apps).