Open Stack

> Pros:
> * This process makes sure implementation design for a feature happens
> _before_ the actual implementation. This is very important as it
> potentially saves developers from wasting a lot of time implementing a
> feature in a suboptimal way.
> * By making reviewers explicitly focus on the more essential aspects of
> a feature design, we make sure that the feature gets a better/more
> efficient implementation.
> * We will have a feature design recorded for posterity. I don't think I
> need to explain how useful this can be.

Two more pros:
* Since Horizon often has UI assets (mockups, prototypes, visual designs) that go along with a feature, the spec repo could also be used to document or link to the locations of these assets. 
* It’s also a way for designers who don’t code to contribute to the project, because they can create, review and edit spec docs.