code: function preprocessor (css, options) {
    if (typeof css !== 'string') {
      throw new Error('sourdough-preprocessor: did not receive a String')
    }
  
    options = options || {}
    options.parser = sugarss
  
    var browserConfig = options.browsers || [
      'Explorer >= 9',
      'last 5 Chrome versions',
      'last 2 Firefox versions',
      'last 2 Safari versions',
      'last 2 iOS versions',
      'Android 4'
    ]
  
    var post = postcss()
  
    // Transform .css files to sss syntax
    function cssToSss (css, file) {
      if (path.extname(file) === '.css') {
        return postcss().process(css, { stringifier: sugarss }).then(function (result) {
          return result.css
        })
      } else {
        return css
      }
    }
  
    // Plugins
    post.use(importer({ transform: cssToSss, root: options.root, onImport: options.onImport }))
    post.use(customProperties())
    post.use(customMedia())
    post.use(mediaRanges())
    post.use(customSelectors())
    post.use(calc())
    post.use(colorFunctions())
    post.use(nested())
    post.use(autoprefixer(browserConfig))
  
    return post.process(css, options)
  }
docs: Process CSS
  
  @param {String} css
  @return {String}