This idea has been merged into another idea. To comment or vote on this idea, please visit WEBFLOW-I-67 Visible custom code inside designer.
Currently the only way to view any custom CSS you add to the site is if you publish your changes. This isn't a very efficient way for users to test out their code as they build pages. We can't even preview the styling changes in the Preview mode either which is what I would expect bare minimum.
You already can do that by including the CSS in an embed code component on the page itself (without any JavaScript). Duplicate of https://wishlist.webflow.com/ideas/WEBFLOW-I-67