postcss-merge-rules 
Merge CSS rules with PostCSS.
Install
With npm do:
npm install postcss-merge-rules --save
Examples
This module will attempt to merge adjacent CSS rules:
By declarations
Input
a {
color: blue;
font-weight: bold
}
p {
color: blue;
font-weight: bold
}
Output
a,p {
color: blue;
font-weight: bold
}
By selectors
Input
a {
color: blue
}
a {
font-weight: bold
}
Output
a {
color: blue;
font-weight: bold
}
By partial declarations
Input
a {
font-weight: bold
}
p {
color: blue;
font-weight: bold
}
Output
a,p {
font-weight: bold
}
p {
color: blue
}
Usage
See the PostCSS documentation for examples for your environment.
Contributing
Pull requests are welcome. If you add functionality, then please add unit tests to cover it.
License
MIT © Ben Briggs