I can't count how many times people asked for a way to copy paste pages/elements from a site to another, and every single time I've said that it must be a difficult thing to do because of CSS merging conflicts. But I'm like everyone, not being able to copy anything from a site to another leaves a strange feeling, like being a caveman in a spaceship : you can do wonders but the simplest thing: nope.
So let put CSS aside and simply keep the blocks structure. Elements, widgets, call them how you like. Being able to copy and paste only that will be a huge time saver sometimes. I design a lot of "cards" lately, little pieces of content with a lot of attention to design, and they contain a ton of divs, blocks, spans, test etc.
And instead of just "copy" and "paste" them, why not "save as a design snippet" and "drag from the "Snippets Library", a library common to all sites in a plan. And we could share them too on our designers pages.
ADMIN EDIT: Vote for this feature here: https://wishlist.webflow.com/ideas/DESIGNER-I-6