Interesting! I have not seen this before. I won't cast too strong an opinion on it just because I haven't had a chance to read about it, but generally speaking it seems like a lateral move. Styles and style modifications need to be articulated somewhere, and whether it's classes or custom attributes doesn't really matter. Or maybe it does; I dunno!

If teams find this way useful, then I'd say go for it. But I'd say to tread lightly simply because it's a pretty big departure from current conventions. I'm not saying that all new things are bad, but it would be worth weighing pros and cons before adopting a new approach like this.