Vivaldi is on it's way to helping break the web
Breaking changes might happen pretty fast. Current version of Chrome is 62. Your code might being broken by the time Chrome 67 arrives. It almost certainly wouldn’t work in Chrome 100.
That is really a problem of JS and DOM standards, not of Vivaldi browser.
And blame the WhatWG for such addition. And always trust your web apps will work in 20 years.
You are on the way to brings us FUD instead of real background information.