how can you set grunt postCSS options?

Setup postCSS in my gruntfile, primarily as a convenient way to handle autoprefixing and minification, using the following code.

postcss: {
  options: {
    map: false, // inline sourcemaps

    processors: [
      require('autoprefixer-core')({
      browsers: ['last 10 versions', 'ie 9'],
      remove: false,
      map: true,
    }), // add vendor prefixes
    require('cssnano')() // minify the result
  ]
},
style: {
  src: '<%= dirs.sassBuild %>/style.css',
  dest: '<%= dirs.publicCss %>/style.min.css'
},
admin: {
  src: '<%= dirs.sassBuild %>/admin.css',
  dest: '<%= dirs.publicCss %>/admin.min.css'
}

The to my horror, I discovered that its mangling my RGBA values, making them HSLA, converting my carefully crafted REM units to pc, adjusting my z-indexes, and god knows what else.

I understand that these are all "features" of postCSS, none of which I want.

Having had a look at some of the documentation its not obvious to me that a) I can disable this behaviour, or b) how to do it with grunt.

is it possible to take back control of these features specifically?

Solution

The optimisations for CSSnano can be found at: http://cssnano.co/optimisations/.

You can disable some optimisations be setting the option to false. So for instance to disable postcss-calc:

require('cssnano')({calc: false}) // minify the result

You also wrote that your rem values are converted to px values as far as i do understand neither autoprefixer nor cssnano does perform this conversion.