Post CSS era

While the JavaScript community advocates inlines styles, monolithic CSS pre-processors become old fashioned. Today we have multiples modern way to style your websites and web apps. PostCSS, a scalable and modular tool, can help us to facilitate our daily CSS developments. This tool allows your to easily parse and transform CSS base on JavaScript plugins. Its ecosystem can help you to lint your code and to write future proof CSS (using cssnext). But what about inline styles? Are they a good idea? What's the downside of this method? Can we mix both methods? Should we use this today?