gulpfile.js

var postcss = require('gulp-postcss');
var gulp = require('gulp');

gulp.task('css', function () {
  var postcss = require('gulp-postcss');
  var sourcemaps = require('gulp-sourcemaps');

  return gulp.src('src/style05.css')
    .pipe(sourcemaps.init())
    .pipe(postcss([require('precss'), require('autoprefixer')]))
    .pipe(sourcemaps.write('.'))
    .pipe(gulp.dest('dist/'));
});