Style Prototypes are a Pattern Library tool for designing in browser. Style prototyping is a technique used to create a typical instance of a design from which a final site can be assembled. Style Prototypes are a unique tool because, not only are they responsive by default (meaning your client will be able to sign off on styles they've been able to see natively on all browsers and devices), they encourage Style and Color Guide driven Style Tile and Component Guide generation. This means that after you've built out your Style Tile, you'll be on your way to having finished your Style Guide, which you need to for Style Guide Driven Design (and coincidentally takes lots of design decisions off of the shoulders of Front End Developers). You'll also never need to have someone guess at what colors they can use with a fully built out Color Guide with both hex and Sass values.