How does Magento 2 work with CSS grunt-autoprefixer

Question

From looking at a fresh install of Magento 2 (v2.1.8) I see from their Gruntfile.js and package.json files that Magento is using grunt-autoprefixer

1. This is great, CSS autoprefixers are really useful. But I can't see how to use it with Magento's Gruntfile, does anyone have an idea how it works?
2. Also, how would this work in production mode on a live environment? Doesn't Magento2 use use a PHP LESS compiler in production mode, while Grunt is just used for developing with.

Gruntfile.js

/**
 * Production preparation task.
 */
prod: function (component) {
    var tasks = [
        'less',
        'autoprefixer',
        'cssmin',
        'usebanner'
    ]

package.json

"devDependencies": {
    "glob": "^5.0.14",
    "grunt": "^0.4.5",
    "grunt-autoprefixer": "^2.0.0",

I ran grunt autoprefixer and it does not seem to work either.

$ grunt autoprefixer
Running "autoprefixer:setup" (autoprefixer) task
Autoprefixer's process() method is deprecated and will removed in next major release. Use postcss([autoprefixer]).process() instead
File setup/pub/styles/setup.css created.

Running "autoprefixer:updater" (autoprefixer) task
Warning: No source files were found. Use --force to continue.

Aborted due to warnings.


Execution Time (2017-10-29 11:12:01 UTC-0)
loading tasks               145ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 30%
loading grunt-autoprefixer  118ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 24%
autoprefixer:setup          216ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 45%
autoprefixer:updater          5ms  ▇▇ 1%
Total 485ms

Answer 1

1. Customize dev/tools/grunt/configs/autoprefixer.json and run grunt autoprefixer.
2. You need to setup it manually in you the deployment pipeline, b/c there is no way to run Grunt tasks from Magento.

Answer 2

Please perform above changes, I hope it will work out well.

package.json

{
  "name": "Project",
  "author": "Vendor",
  "description": "Node modules dependencies for local development",
  "version": "2.0.0",
  "license": "(OSL-3.0 OR AFL-3.0)",
  "repository": {
    "type": "git",
    "url": "https://github.com/magento/magento2.git"
  },
  "homepage": "http://magento.com/",
  "devDependencies": {
    "autoprefixer": "^7.1.1",
    "glob": "^5.0.14",
    "grunt": "^0.4.5",
    "grunt-banner": "^0.4.0",
    "grunt-contrib-clean": "^0.6.0",
    "grunt-contrib-connect": "^0.9.0",
    "grunt-contrib-cssmin": "^0.10.0",
    "grunt-contrib-imagemin": "^0.9.2",
    "grunt-contrib-jasmine": "^0.8.1",
    "grunt-contrib-less": "^0.12.0",
    "grunt-contrib-watch": "^0.6.1",
    "grunt-eslint": "17.3.1",
    "grunt-exec": "^0.4.6",
    "grunt-jscs": "2.2.0",
    "grunt-postcss": "^0.8.0",
    "grunt-replace": "^0.9.2",
    "grunt-styledocco": "^0.1.4",
    "grunt-template-jasmine-requirejs": "^0.2.3",
    "grunt-text-replace": "^0.4.0",
    "imagemin-svgo": "^4.0.1",
    "load-grunt-config": "^0.16.0",
    "morgan": "^1.5.0",
    "node-minify": "^1.0.1",
    "path": "^0.11.14",
    "serve-static": "^1.7.1",
    "strip-json-comments": "^1.0.2",
    "time-grunt": "^1.0.0",
    "underscore": "^1.7.0"
  },
  "engines": {
    "node": ">=0.10.0"
  }
}

postcss.js

/**
 * PostCSS autoprefixer initialisation
 *
 * Docs: https://github.com/postcss/autoprefixer
 * Config: dev/tools/grunt/configs/postcss.json
 * Usage: grunt autoprefixer:themename [--verbose] [--debug]
 * To do: load src from themes.js
 *
 * @param grunt
 */

module.exports = function(grunt) {
    grunt.loadNpmTasks('grunt-postcss');

    grunt.registerTask('autoprefixer', function (target) {
        var currentTarget = target || 'dist';

        /*** configuration tweaks ***/
        var config = grunt.config.get('postcss');

        // set 'processors' options (not possible to set function in json config)
        config['options'].processors = [require('autoprefixer')({browsers: ['last 2 versions']})];

        // apply theme source and destination dynamically
        if (target) {
            config[target] = {
                src: ['pub/static/frontend/*/'+target+'/*/css/*.css']
            };
        }
        grunt.config.set('postcss', config);

        grunt.option('force', true);
        grunt.task.run('postcss:'+currentTarget);
    });
};

postcss.json

{
    "options": {
        "map": true
    },
    "dist": {
      "src": ["pub/static/frontend/*/*/*/css/*.css"]
    }
}

After setup above files, run grunt autoprefixer

Answer 3

To be really clear for everyone: postcss.js & postcss.json - go in /dev/tools/grunt/configs package.json - in the website root - just check it includes autoprefixer under devDependencies

Once done run exec / less / watch / setup in the usual way.

Then access your website in a browser to create the CSS cache files.

then, run: grunt autoprefixer

this will check the resulting css files and add -webkit- and other browser support

This is a good fix for development but I have not found a way to use this in production.