Appears to use Chromium by default, and also ship WebKit to toggle for individual websites: https://crestbrowser.com/guides/engines/

Of course this comment section also warrants a mention of Zen Browser, another Arc alternative that's been going for a while already, is based on Firefox, and is available cross-platform: https://zen-browser.app

I am sort of working on the same thing. It is mostly for my own use so far, I might share it at some point.

What I’ve focused on besides the basic features of the arc sidebar is memory usage.

I am a tab power user, in Brave I had 370 tabs “open”. Each tab still takes resources when asleep though. But I use these tabs more as bookmarks. So they are asleep 99.99% of the time, if not more.

The browser I’ve been working on looks a lot like Crest because they both take Arc as a starting point. I am interested to see where they both move towards from there.

Most of these features are in Firefox. Vertical tabs, tab groups/containers, split view, etc.

I don't get the appeal of the Arc-esque browsers. They look very clean in screenshots, but I tried both Arc and Zen and never got to feel productive with them. Can someone enlighten me?

Finally. Most important features that kept me from switching to chrome, brave or any other browser was: 1. The sidebar with tabs that shares the same state over multiple windows. 2. The preview windows that open directly where you open a link.

Also just wanna get in here and promote Zen, i love it. But the crest and design things here seem nice

The dual engine approach is interesting, never seen that