In reply to Steven Stern.

Having just worked on a site that had 5 Custom CSS plugins installed... 3 of which were actively in use... centralizing this all into a single core feature makes a lot of sense to me.

For those wondering, 5: 1 built into the theme, 1 from VIsual Composer extension, 1 from JetPack (module enabled), 2 other stand alone custom CSS plugins installed by who knows.

Personally, I could care less about the customizer front end previewing aspect. I get some people do, but I think the win is from separating CSS editing from PHP editing (the later I'd just as happily see permanently removed) and of building this in as a core feature to get away from the fractured custom CSS experience I mentioned above.

I think it of a bit like the custom menus getting built in after a bunch of conflicting custom menu plugins became popular.