> I think the most important job for a browser is to stay in the background and let the user focus on the website.

Actually, this is my favorite part about Arc. Most of the time when using Arc I'm working in fullscreen mode with 100% of the UI hidden. There is literally zero on my screen except the page itself. If I mouseover to the left (or hit cmd-S) I can see the sidebar/addressbar etc.

It's absolutely wonderful and really puts the focus on the content. I find it much better than how Chrome handles fullscreen.