Remove Unused CSS from WordPress Automatically with Grunt. See http://cfxdesign.com/remove-unused-css-from-roots-theme-automatically-with-grunt/

add-exec-and-uncss-tasks-to-package.json

{
  "name": "roots",
  "version": "7.0.0",
  "author": "Ben Word <ben@benword.com>",
  "homepage": "http://roots.io",
  "repository": {
    "type": "git",
    "url": "git://github.com/roots/roots.git"
  },
  "bugs": {
    "url": "https://github.com/roots/roots/issues"
  },
  "licenses": [
    {
      "type": "MIT",
      "url": "http://opensource.org/licenses/MIT"
    }
  ],
  "scripts": {
    "postinstall": "node_modules/.bin/bower install"
  },
  "engines": {
    "node": ">= 0.10.0"
  },
  "devDependencies": {
    "bower": "~1.3.8",
    "grunt": "~0.4.5",
    "grunt-autoprefixer": "~0.8.1",
    "grunt-contrib-concat": "~0.4.0",
    "grunt-contrib-jshint": "~0.10.0",
    "grunt-contrib-less": "~0.11.3",
    "grunt-contrib-uglify": "~0.5.0",
    "grunt-contrib-watch": "~0.6.1",
    "grunt-exec": "~0.4.6",
    "grunt-modernizr": "~0.5.2",
    "grunt-uncss": "~0.3.5",
    "grunt-wp-assets": "~0.2.6",
    "load-grunt-tasks": "~0.6.0",
    "time-grunt": "~0.4.0"
  }
}

complete-less-task.js

    less: {
      dev: {
        files: {
          'assets/css/main.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: false,
          // LESS source map
          // To enable, set sourceMap to true and update sourceMapRootpath based on your install
          sourceMap: true,
          sourceMapFilename: 'assets/css/main.css.map',
          sourceMapRootpath: '/app/themes/roots/'
        }
      },
      build: {
        files: {
          'assets/css/main.min.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: true
        }
      },
      build_uncss: {
        files: {
          'assets/css/main.min.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: false
        }
      },
      compress: {
        files: {
          'assets/css/main.min.css': [
            'assets/css/main.min.css'
          ]
        },
        options: {
          compress: true
        }
      }
    },

get_all_wp_publish.php

<?php

function show_sitemap() {
  if(WP_ENV === 'development' && isset($_GET['show_sitemap'])) {
    $the_query = new WP_Query(array('post_type' => 'any', 'posts_per_page' => '-1', 'post_status' => 'publish'));
    $urls = array();
    while($the_query->have_posts()) {
      $the_query->the_post();
      $urls[] = get_permalink();
    }
    die(json_encode($urls));
  }
}
add_action('template_redirect', 'show_sitemap');

?>

grunt-exec-config.js

    exec: {
      get_grunt_sitemap: {
        command: 'curl --silent --output sitemap.json '+domain+'?show_sitemap'
      }
    },

grunt-uncss-config.js

    uncss: {
      dist: {
        options: {
          ignore       : [/expanded/,/js/,/wp-/,/align/,/admin-bar/],
          stylesheets  : ['assets/css/main.min.css'],
          ignoreSheets : [/fonts.googleapis/],
          urls         : [], //Overwritten in load_sitemap_and_uncss task
        },
        files: {
          'assets/css/main.min.css': ['**/*.php']
        }
      }
    }

Gruntfile.js

'use strict';
module.exports = function(grunt) {
  // Load all tasks
  require('load-grunt-tasks')(grunt);
  // Show elapsed time
  require('time-grunt')(grunt);

  var jsFileList = [
    'assets/vendor/bootstrap/js/transition.js',
    'assets/vendor/bootstrap/js/alert.js',
    'assets/vendor/bootstrap/js/button.js',
    'assets/vendor/bootstrap/js/carousel.js',
    'assets/vendor/bootstrap/js/collapse.js',
    'assets/vendor/bootstrap/js/dropdown.js',
    'assets/vendor/bootstrap/js/modal.js',
    'assets/vendor/bootstrap/js/tooltip.js',
    'assets/vendor/bootstrap/js/popover.js',
    'assets/vendor/bootstrap/js/scrollspy.js',
    'assets/vendor/bootstrap/js/tab.js',
    'assets/vendor/bootstrap/js/affix.js',
    'assets/js/plugins/*.js',
    'assets/js/_*.js'
  ];

  var domain = 'http://example.dev/';

  grunt.initConfig({
    jshint: {
      options: {
        jshintrc: '.jshintrc'
      },
      all: [
        'Gruntfile.js',
        'assets/js/*.js',
        '!assets/js/scripts.js',
        '!assets/**/*.min.*'
      ]
    },
    less: {
      dev: {
        files: {
          'assets/css/main.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: false,
          // LESS source map
          // To enable, set sourceMap to true and update sourceMapRootpath based on your install
          sourceMap: true,
          sourceMapFilename: 'assets/css/main.css.map',
          sourceMapRootpath: '/app/themes/roots/'
        }
      },
      build: {
        files: {
          'assets/css/main.min.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: true
        }
      },
      build_uncss: {
        files: {
          'assets/css/main.min.css': [
            'assets/less/main.less'
          ]
        },
        options: {
          compress: false
        }
      },
      compress: {
        files: {
          'assets/css/main.min.css': [
            'assets/css/main.min.css'
          ]
        },
        options: {
          compress: true
        }
      }
    },
    concat: {
      options: {
        separator: ';',
      },
      dist: {
        src: [jsFileList],
        dest: 'assets/js/scripts.js',
      },
    },
    uglify: {
      dist: {
        files: {
          'assets/js/scripts.min.js': [jsFileList]
        }
      }
    },
    autoprefixer: {
      options: {
        browsers: ['last 2 versions', 'ie 8', 'ie 9', 'android 2.3', 'android 4', 'opera 12']
      },
      dev: {
        options: {
          map: {
            prev: 'assets/css/'
          }
        },
        src: 'assets/css/main.css'
      },
      build: {
        src: 'assets/css/main.min.css'
      }
    },
    modernizr: {
      build: {
        devFile: 'assets/vendor/modernizr/modernizr.js',
        outputFile: 'assets/js/vendor/modernizr.min.js',
        files: {
          'src': [
            ['assets/js/scripts.min.js'],
            ['assets/css/main.min.css']
          ]
        },
        uglify: true,
        parseFiles: true
      }
    },
    version: {
      default: {
        options: {
          format: true,
          length: 32,
          manifest: 'assets/manifest.json',
          querystring: {
            style: 'roots_css',
            script: 'roots_js'
          }
        },
        files: {
          'lib/scripts.php': 'assets/{css,js}/{main,scripts}.min.{css,js}'
        }
      }
    },
    watch: {
      less: {
        files: [
          'assets/**/*.less'
        ],
        tasks: ['less:dev', 'autoprefixer:dev']
      },
      js: {
        files: [
          jsFileList,
          '<%= jshint.all %>'
        ],
        tasks: ['jshint', 'concat']
      },
      livereload: {
        // Browser live reloading
        // https://github.com/gruntjs/grunt-contrib-watch#live-reloading
        options: {
          livereload: false
        },
        files: [
          'assets/css/main.css',
          'assets/js/scripts.js',
          'templates/*.php',
          '*.php'
        ]
      }
    },
    exec: {
      get_grunt_sitemap: {
        command: 'curl --silent --output sitemap.json '+domain+'?show_sitemap'
      }
    },
    uncss: {
      dist: {
        options: {
          ignore       : [/expanded/,/js/,/wp-/,/align/,/admin-bar/],
          stylesheets  : ['assets/css/main.min.css'],
          ignoreSheets : [/fonts.googleapis/],
          urls         : [], //Overwritten in load_sitemap_and_uncss task
        },
        files: {
          'assets/css/main.min.css': ['**/*.php']
        }
      }
    }
  });

  // Register tasks
  grunt.registerTask('default', [
    'dev'
  ]);
  grunt.registerTask('dev', [
    'jshint',
    'less:dev',
    'autoprefixer:dev',
    'concat'
  ]);
  grunt.registerTask('build', [
    'jshint',
    'less:build',
    'autoprefixer:build',
    'uglify',
    'modernizr',
    'version'
  ]);
  grunt.registerTask('load_sitemap_json', function() {
    var sitemap_urls = grunt.file.readJSON('./sitemap.json');
    grunt.config.set('uncss.dist.options.urls', sitemap_urls);
  });
  grunt.registerTask('build_uncss', [
    'jshint',
    'less:build_uncss',//compile LESS without compression
    'exec:get_grunt_sitemap',//get sitemap
    'load_sitemap_json',//load sitemap into uncss options
    'uncss:dist',//run uncss (write results back into main.min.css)
    'less:compress',//compress main.min.css
    'autoprefixer:build',//finish remaining build tasks as before
    'uglify',
    'modernizr',
    'version'
  ]);
};

regster-tasks-Gruntfile.js

  grunt.registerTask('load_sitemap_json', function() {
    var sitemap_urls = grunt.file.readJSON('./sitemap.json');
    grunt.config.set('uncss.dist.options.urls', sitemap_urls);
  });
  grunt.registerTask('build_uncss', [
    'jshint',
    'less:build_uncss',//compile LESS without compression
    'exec:get_grunt_sitemap',//get sitemap
    'load_sitemap_json',//load sitemap into uncss options
    'uncss:dist',//run uncss (write results back into main.min.css)
    'less:compress',//compress main.min.css
    'autoprefixer:build',//finish remaining build tasks as before
    'uglify',
    'modernizr',
    'version'
  ]);