It's broken in the exact same way in the last version of Vivaldi that works on Windows <10.

From a quick look at the second horizontal strip that starts "what is this?", and without wasting time doing too much digging, it appears to be ultimately caused by using a grid layout for a heading followed by a few paragraphs, all of which fill the available (constrained) width, which amounts to one of the absolute simplest layouts possible in CSS - block. Last time I saw something similar, albeit with flexbox not grid, it was from a junior developer who appeared to have only learned flexbox and nothing else.

As a quick semi-fix, just disabling grid-template-columns on .base-layout is enough to fix almost everything except the constrained width.