A friend with personal aversion to Mac OS 10.10 GUI elements.
Hi, Writing as a friend, I would sincerely like to test your browser. However, unfortunately, we see that there is only one version available and that version has been constructed with the new Mac OS GUI elements. As we do not like the new GUI at all and refuse to upgrade our OS beyond the point where the 'old' GUI elements stopped being used, Mac OS 10.9.5, we humbly ask if your browser could be made available, to friends, in a non-v10.10-GUI-elements interface. Kind regards.
To which OSX-specific GUI elements are you referring? The close, minimize, and maximize/restore buttons? Scrollbars? (Note: Vivaldi lets you switch between native title bar and the more compact option of having the close, minimize, and maximize/restore buttons included at the left end of the tab bar. Settings > Appearance > Window Appearance > ☑Use Native Window)
Aside from window management stuff like that, pretty much of the rest of the UI is fairly consistent between OSes. (Although I've noticed an oddity that the tab-close "X"s appear on the right end of tabs in Vivaldi for Linux, instead of replacing the left-placed favicon on hover like they do on OS X. I'm not sure if that inconsistency is by design or what.)