work.suroh.tk/node_modules/bs-recipes/recipes/grunt.sass.autoprefixer/desc.md

This example shows how you can chain potentially slow-running tasks, but still achieve CSS Injection. The trick, as seen below, is to use the bsReload task that now comes bundled with grunt-browser-sync since 2.1.0

Don't forget the spawn: false option for the watch task - it's a requirement that allows Browsersync to work correctly

watch: {
    options: {
        spawn: false // Important, don't remove this!
    },
    files: 'app/**/*.scss',
    tasks: ['sass', 'autoprefixer', 'bsReload:css']
},