Maybe only developers would write CSS from scratch, but there are plenty of non-devs who are capable of finding CSS snippets (e.g. on userstyles.org) and adding it to some sort of user stylesheet. However, once they're lost once the process gets beyond "find your user stylesheet at this path" or "click to install with Stylish."

Are you implying that "non-developers" should be discouraged from doing so? Would you agree if CSS was replaced with HTML?

Consider that many people learned how to write their own webpages with nothing more than a browser and a text editor, and the simplicity and soft error recovery characteristics of HTML is what I think was responsible for a large part of the Internet's growth. The barrier to entry was low, so it was easy to participate. Many people I know learned how to create webpages this way, largely by example (i.e. view source). Many of them also learned a bit of CSS too. These users produced and consumed content. But now, all I see is a growing divide between "users" and "developers", with these changes in browsers both reflecting and encouraging it, and I can't see that as being at all a good thing.