Rework vs Pleeease

Through transforms you can modify existing properties to give them new attributes and options, so instead of managing messy mixins, you can add a simple new attribute where they make the most sense.

Because Rework plugins are done in code, there are no limits to what they can do, and they tend to provide more advanced functionality that would be harder to implement in other preprocessors, such as file I/O and custom logic.

Rework isn't a language for compiling to CSS but rather a library around parsing it and transforming it. For example, a vendor prefix plugin will inject prefixes around needed properties so you don't have to muddy up your CSS dealing with it.

Because Rework is built around plugins at its core, it makes for easier plugin writing if you find you want to add in new functionality.

Although you don't have to, since Rework works on vanilla CSS, you could use another preprocessor that has a syntax you enjoy more before applying Rework's transforms.

Rework plugins can recognize custom properties and transform them via plugins. This allows you to keep your CSS clean and expand its functionality in a native feeling way, without having to learn a bunch of new language constructs.

preprocess CSS (experimental)
adds prefixes, based on Autoprefixer
provides fallbacks for rem unit, CSS3 pseudo-elements notation
adds opacity filter for IE8
converts CSS shorthand filters to SVG equivalent
packs same media-query in one @media rule
inlines @import styles
minifies the result
generates sourcemaps from pre- to postprocessors

If you have repeated media queries in your stylesheet, Pleeease will pack them into a single media query when compiled.

Rem unites are not supported in IE8 and below, so Pleeease provides a pixel fallback.

Pleeease uses Autoprefixer to add vendor prefixes based on which browsers you want to support (prefixes are added based on information from caniuse.com.