<blockquote>What we need [instead] is to deliver the minimum CSS required to make the page layout viable. It's impossible to provide one size fits all guidelines as different sites will have different needs. However, this baseline CSS is likely to include the core typography, colours and layout.

I appreciate this isn't easy - it's not something you are likely to be able to do with a Grunt plugin, you are going to have to have an in-depth understanding of your visual design and your CSS and how the two are interrelated. You will need to know what is the absolute minimal core of your visual design and how that is described by CSS. From this you can deliver the CSS that describes this core and the enhanced CSS that completes the visual design.</blockquote>