Hi @dbtdbtdbt - Welcome to the forum. That is a great question. Some significant CSS functionalities are missing from the designer. CSS calc is a big one; another is no support for descendant selectors. Then there are CSS variables which would be a huge help. The lack of these alone creates many extra workflow and styling issues, leading to poor maintainability. Webflow has decided to instead focus on components.
In the Wishlist, you will see Support for CSS calc() | Webflow Wishlist, with over 1200 votes. You could add yours.
Here is one for CSS Variables which needs some love - Support for CSS variables & binding styles to the | Webflow Wishlist