The problem is that CSS is serving multiple masters. It is specifying how the designer is wanting a page to be laid out while doing do in a way that conforms to the way the receiver wants it to be laid out.
I think there is an opportunity for pages to have regions of priority. I thing there would be merit in something where client devices have freedom to decide some parts of the layout but others are ridgidly controlled by designers.
You can kind of do this already with CSS with absolute and relative positioning and calc() for control and flex for, well, flexibility. It's not in a form that particularly facilitates it though. I'd like a better ability to choose whether elements to push the bounds of their containers or whether containers squeeze their content, and what to do when in conflict. scrolling, clipping, exceeding bounds, scaling to fit are all options.