I don't know if this is best practice, but it works (without using gulp-watch or any other package)

const gulp = require('gulp');


function css(cb){
  const postcss = require('gulp-postcss')
  
  cb();
  return gulp.src('src/style.css')
    // ...
    .pipe(postcss([
      // ...
      require('autoprefixer'),
      // ...
    ]))
    // ...
    .pipe(gulp.dest('./src'))

}

gulp.task('default', function() {
  gulp.watch(['./src/style.css'], {ignoreInitial: false}, css)
})

The key is to use gulp.watch inside a function and use the first argument of the watch function as a callable.

@AeonFr that's not quite right. I've updated the examples in that tutorial article so they all export a default task.

Thanks @phated.

The key point is to call the cb() callback function as soon as the process is finished, yet in my code right now the callback is called immediately when the function starts. This looks as if the process finishes in a breeze.

Just for the sake of learning (bc right now it's working), how would one use the cb() callback properly?

It would be nice if the documentation had at least one example with a proper exports.default and a body that finishes with the cb(). It's not necessary to include it in all of the examples (but I guess it doesn't hurt nobody).

Again, thanks for replying.
Loving gulp so far ♥

@AeonFr The Async Completion guide explains the usage of an error-first callback (if you aren't able to return a stream, but you are already doing that so you don't need to use the callback).

Got it! Thanks, removing cb() does the trick!