postcss：BrowserSync与gulp不兼容，在检测到更改时未应用HTML和CSS

Question

var gulp            = require('gulp');
var browserSync     = require('browser-sync');
var reload          = browserSync.reload;
var postcss         = require('gulp-postcss');
var atImport        = require("postcss-import");
var nested          = require('postcss-nested');
var autoprefixer    = require('autoprefixer-core');
var cssnext         = require('cssnext');
var precss          = require('precss');
var nano            = require('gulp-cssnano');
var rename          = require('gulp-rename');
var cp              = require('child_process');
var jade            = require('gulp-jade');
var plumber         = require('gulp-plumber');
var uglify          = require('gulp-uglify');
var concat          = require('gulp-concat');


var paths = {
  'bower': './bower_components/',
  'assets': './assets/'
};

var messages = {
    jekyllBuild: '<span style="color: grey">Running:</span> $ jekyll build'
};

gulp.task('jekyll-build', function (done) {
    browserSync.notify(messages.jekyllBuild);
    return cp.spawn('jekyll', ['build'], {stdio: 'inherit'})
        .on('close', done);
});

gulp.task('jekyll-rebuild', ['jekyll-build'], function () {
    browserSync.reload();
});

gulp.task('browser-sync', ['styles', 'jekyll-build'], function() {
    browserSync({
        server: {
            baseDir: '_site'
        }
    });
});

gulp.task('styles', function() {
  var processors = [
    atImport,
    cssnext({}),
    precss({}),
    nested,
    autoprefixer
  ];

  return gulp.src('assets/css/main.css')
    .pipe(postcss(processors))
    .pipe(gulp.dest('_site/assets/css'))
    .pipe(nano())
    .pipe(rename({suffix: '.min'}))
    .pipe(gulp.dest('assets/css'))
    .pipe(browserSync.reload({stream: true}));
});

gulp.task('scripts', function() {
  gulp.src([
    paths.bower + '/jquery/dist/jquery.js',
    paths.bower + '/Materialize/dist/js/materialize.js',
    paths.assets + '/js/main.js'
  ])
  .pipe(plumber())
  .pipe(concat('main.js'))
  .pipe(rename({suffix: '.min'}))
  .pipe(uglify())
  .pipe(gulp.dest('assets/js'));

  return gulp.src([
    paths.bower + '/modernizr/modernizr.js'
  ])
    .pipe(gulp.dest('assets/js'))
    .pipe(reload({stream: true}));
});

gulp.task('jade', function() {
  return gulp.src('_jadefiles/*.jade')
  .pipe(jade())
  .pipe(gulp.dest('_includes'));
});

gulp.task('watch', function () {
    gulp.watch('assets/css/**', ['styles']);
    gulp.watch('assets/js/**', ['jekyll-rebuild']);
    gulp.watch(['index.html', '_layouts/*.html', '_posts/*'], ['jekyll-rebuild']);
    gulp.watch('_jadefiles/*.jade', ['jade']);
});

gulp.task('default', ['browser-sync', 'scripts', 'jade', 'watch']);

这个gulpfile.js的灵感主要来自使用Jade和Jekyll的[ShakyShane] [1]。

我是gulp.js的新手，想要应用postcss来增强我当前的工作流程，但是即使在终端中检测到它们也不会在浏览器中发生变化。希望你们能帮忙。