Multiple CSS properties of same name about garden HOT 2 CLOSED

Garden uses a hash map to express rules, and hash maps cannot repeat keys.
...
This assumes that the order doesn't matter, but by using hash maps in the first place we're kind of assuming that CSS rule order doesn't matter within a single selector.

Both of these points are certainly valid and I knew it would be important to have solutions for them. For Garden, the solution is actually quite simple: use multiple hash maps.

[:div
 {:background "rgb(...)"}
 {:background "-moz-linear-gradient(...)"}
 {:background "-webkit-gradient(...)"}]

This will give you the output you are looking for.

div {
  background: rgb(...);
  background: -moz-linear-gradient(...);
  background: -webkit-gradient(...);
}

Now, I should point out that I hadn't considered this case in particular when thinking about the data structure semantics and I really like your idea of using sets as another solution. Since there are no semantics regarding sets within the context of a CSS declaration, I am inclined to included them based on your example. Ordering could still pose a small issue for some authors but, again, it's possible to use multiple maps to achieve the same end. Other than that I can't think of any good reasons not to permit sets to be used as you've demonstrated. In fact, I'll probably add it to the TODO list shortly. :)

It could also be useful to have a way to introduce a raw CSS string into a garden data structure...

I've thought a lot about this as well. While it could be nice to have strings around as a "backdoor", I'm not totally convinced it's a good idea. In fact, I've tried to design Garden such that it would eliminate any potential reasons for using raw CSS strings at all. With Hiccup, the case for strings as you've shown them has clear and obvious benefits. With Garden, I'm not so sure. However, I'm always open to solid cases and listening to what people have to say.

from garden.

It could also be useful to have a way to introduce a raw CSS string into a garden data structure, similar to how in hiccup you can mix and match:

(use '[hiccup.core :refer [html]])
(html [:div "<p>this is raw html</p>"])
; Result: "<div><p>this is raw html</p></div>"

from garden.