You treat your stylesheet as your style guide, design system, or single source of truth. That's where you define your fonts, colors scales etc. So you actually

You treat your stylesheet as your style guide, design system, or single source of truth. That's where you define your fonts, colors scales etc. So you actually don't have to think about specific widths while you're building, you reference a width that is already set up in your stylesheet! I find that it improves the design to dev workflow. You experiment with design in a design tool like Sketch, and then translate your final idea or rules into a stylesheet, which is the basis for your live prototyping in editor & browser (wrapping HTML elements and referencing styles in your stylesheet).