When comparing PostCSS vs Cirrus, the Slant community recommends Cirrus for most people. In the question“What are the best minimal CSS frameworks?” Cirrus is ranked 9th while PostCSS is ranked 12th. The most important reason people chose Cirrus is:
Supports both Flexbox and CSS grid making it a great modern choice for designing web apps.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Fast
PostCSS is 3-30 times faster than Sass (including libsass), Less, and Stylus
Pro Flexible
PostCSS allows you to opt-in to the features you need with plugins. This allows you to set it up to behave exactly like Sass, with nesting, mixing, extends, and more. On the other hand, it allows you to use plugins by themselves for things like auto-prefixing, minification, and more. You can even set up your own custom "stack" of plugins to do exactly what you like.
Pro Doesn't force designers to learn a new syntax
Rather than learn a different syntax, PostCSS allows you to write in pure CSS.
Pro JavaScript-based out of the box
Since it's basically CSS extended through JavaScript it works in the browser directly without the need to compile it beforehand.
Pro Supports Flexbox and CSS Grid
Supports both Flexbox and CSS grid making it a great modern choice for designing web apps.
Pro CSS Only
No additional JS/jQuery required to use.
Pro Mobile Responsive
Extremely responsive and supports many smaller screens.
Pro Open Source
Open source and quite actively maintained on Github.
Pro Lightweight
Much smaller than Bootstrap with just as much flexibility.
Cons
Con Harder to install and keep working
The immense flexibility of PostCSS plus its current rapid evolution makes it harder to install, configure and keep running than the more monolithic and mature preprocessors.
Con Outdatet, plugins are often based on different postcss versions and don't work together properly
Con Some plugins need to run in a certain order
Some plugins can only work if initialized after some other plugins. For example, transforming and applying CSS variables needs to run before running a plugin which uses these variables inside conditional transformations.