Sounds like that older Vivaldi version does not support CSS nesting. I looked into it. I'm assuming you are using Vivaldi 5.6 which uses Chromium 108, native CSS nesting became supported in Chromium 112.

The layout approach is based on this Kevin Powell video: https://m.youtube.com/watch?v=c13gpBrnGEw

It uses CSS Grid to create a centered, width-constrained content area while allowing individual elements to break out of it or span the full viewport width. This provides a flexible, consistent layout without the need for additional wrapper elements in the HTML.

CSS nesting has had full browser support for more than 3 years now, I recommend updating. I'll also add fallback for older browsers, but the Grid layout itself isn't the issue.