Browsersync not working?

Question

HTML

Getting to know gulp. At first everything worked properly, after a while it stopped. Where could I have made a mistake?

var gulp        = require ('gulp'),
    sass        = require ('gulp-sass'),
    browserSync = require ('browser-sync'),
    concat      = require ('gulp-concat'),
    uglify      = require ('gulp-uglifyjs'),
    cssnano     = require ('gulp-cssnano'),
    rename      = require ('gulp-rename'); 

gulp.task('sass', function() {
  return gulp.src('app/sass/**/*.sass')
  .pipe(sass())
  .pipe(gulp.dest('app/css'))
  .pipe(browserSync.reload({stream: true}));
});

gulp.task('scripts', function () {
    return gulp.src([
      'app/libs/jquery/dist/jquery.min.js',
      'app/libs/magnific-popup/dist/jquery.magnific-popup.min.js',
    ])
    .pipe(concat('libs.min.js'))
    .pipe(uglify())
    .pipe(gulp.dest('app/js/'));
});

gulp.task('css-libs', ['sass'], function () {
    return gulp.src('app/css/libs.css')
    .pipe(cssnano())
    .pipe(rename({suffix: '.min'}))
    .pipe(dulp.dest('app/css'));
});

gulp.task('browser-sync', function () {
    browserSync({
    server: {
    	baseDir: 'app'
    },
    notify: false
    });
});

gulp.task('watch', ['browser-sync', 'css-libs', 'scripts'], function () {
  gulp.watch('app/sass/**/*.sass', ['sass']);
  gulp.watch('app/*.html', browserSync.reload);
  gulp.watch('app/js/**/*.js', browserSync.reload);
});

Answer 1

it's a typo ?