title: How to combine postcss / autoprefixer with LESS
body: <p>I have a site for which I use custom LESS stylesheets and they include Bootstrap LESS stylesheets. It is all compiled to the final <code>style.css</code> file with a <code>style.css.map</code> sourcemap.</p>
  
  <p>This is how my Gulpfile looks:</p>
  
  <pre><code>var gulp = require('gulp');
  var less = require('gulp-less-sourcemap');
  
  var mySite = '.';
  
  gulp.task('less', function () {
      gulp.src(mySite + '/css/**.less')
        .pipe(less())
        .pipe(gulp.dest(mySite + '/css'));
  });
  </code></pre>
  
  <p>Now I'd like to add <a href="https://github.com/postcss/autoprefixer" rel="nofollow">autoprefixer</a>. This is what I've tried but it doesn't work:</p>
  
  <pre><code>var gulp = require('gulp');
  var less = require('gulp-less-sourcemap');
  var postcss = require('gulp-postcss');
  var autoprefixer = require('autoprefixer-core');
  
  var mySite = '.';
  
  gulp.task('less', function () {
      gulp.src(mySite + '/css/**.less')
        .pipe(less())
        .pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
        .pipe(gulp.dest(mySite + '/css'));
  });
  </code></pre>
  
  <p>It's because I am piping the generated sourcemaps into <code>postcss</code> and it cannot handle it. So I've tried this:</p>
  
  <pre><code>var gulp = require('gulp');
  var less = require('gulp-less-sourcemap');
  var postcss = require('gulp-postcss');
  var autoprefixer = require('autoprefixer-core');
  
  var mySite = '.';
  
  gulp.task('less', function () {
      gulp.src(mySite + '/css/**.less')
        .pipe(less())
        .pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
        .pipe(gulp.dest(mySite + '/css'));
  
  
      gulp.src(mySite + '/css/**.css')
        .pipe(postcss([autoprefixer({ browsers: ['last 2 version'] })]))
        .pipe(gulp.dest(mySite + '/css'));
  });
  </code></pre>
  
  <p>However, the prefixes are still not added. Maybe it's because the Bootstrap's CSS is already prefixed and it's somehow a problem for <code>postcss</code>?</p>
  
  <p>How to make this work?</p>