gulp-sassとgulp-postcssで快適コンパイル環境を構築 ref: http://qiita.com/youthkee/items/f456fb5730655cc8f9c2

file0.txt

$ npm i gulp gulp-sass gulp-postcss postcss-cssnext

file2.txt

$ gulp scss

gulpfile.js

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var cssnext = require('postcss-cssnext');
var PostcssSimpleVars = require('postcss-simple-vars');
var PostcssNested = require('postcss-nested');
var PostcssMixins = require('postcss-mixins');
var PostcssImport = require('postcss-import');

var paths = {
  'src': 'src/',
  'dist': 'dist/'
}

gulp.task('css', function() {
  var processors = [
      cssnext(),
      PostcssMixins(),
      PostcssSimpleVars(),
      PostcssNested(),
      PostcssImport(),
  ];
  return gulp.src(paths.src + 'css/*.css')
    .pipe(postcss(processors))
    .pipe(gulp.dest(paths.dist + 'css/'))
});

style.css

.header {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
}

style.scss

.header {
  display: flex;
}

syule.scss

.header {
  display: flex;
}