For the purpose of writing the CSS code, the pixel image dimensions shown by the design’s redlines were useless. They would only ever work in scenarios where the user’s viewport happened to match the designer’s canvas size — for any other viewport size they were incorrect. In all likelihood, the designer had laid out the images as desired and then measured them for the annotations.
However, such decisions are usually still buried in some file in some repo used by developers working on the web-based product they own. What about designers? Other web products? Across to other platforms like iOS and Android? We’ve encoded decisions meant for everyone, but they owned in some dungeon of a repo nobody else can find.