CSS tree shaking

[kybishop]

As per https://vimeo.com/172508586

Make a list of the attributes and elements used during build time; only include the relevant styles.

Will spike an RFC of this soon with thoughts on how to implement. The only tricky bit is where to store the intermediate list such that the scss compiler can get to it.

[kybishop]

Shifting focus to the long-term approach: https://github.com/html-next/optimizing-css-compiler

[RobbieTheWagner]

Should we close this then @kybishop?

[kybishop]

Let's keep it open incase anyone else is curious about what we're doing regarding tree shaking, since it is mentioned in some of the talks on Flexi.

I'll eventually create a master issue regarding the CSS optimizer once I understand all the moving pieces we need to deal with to incorporate it into Flexi.