Sweetalert
SweetAlert automatically centers itself on the page and looks great no matter if you're using a desktop computer, mobile or tablet. It's even highly customizeable, as you can see below!
  swal({
    title: "Are you sure?",
    text: "You will not be able to recover this imaginary file!",
    icon: 'warning',
    dangerMode: true,
    buttons: {
      cancel: 'No, Please!',
      delete: 'Yes, Delete It'
    }
  }).then(function (willDelete) {
    if (willDelete) {
      swal("Poof! Your imaginary file has been deleted!", {
        icon: "success",
      });
    } else {
      swal("Your imaginary file is safe", {
        title: 'Cancelled',
        icon: "error",
      });
    }
  });
              
          
  swal({
    text: 'Search for a movie. e.g. "La La Land".',
    content: "input",
    button: {
      text: "Search!",
      closeModal: false,
    },
  })
    .then(name => {
      if (!name) throw null;
      return fetch(`https://itunes.apple.com/search?term=${name}&entity=movie`);
    })
    .then(results => {
      return results.json();
    })
    .then(json => {
      const movie = json.results[0];
      if (!movie) {
        return swal("No movie was found!");
      }
      const name = movie.trackName;
      const imageURL = movie.artworkUrl100;
      swal({
        title: "Top result:",
        text: name,
        icon: imageURL,
      });
    })
    .catch(err => {
      if (err) {
        swal("Oh noes!", "The AJAX request failed!", "error");
      } else {
        swal.stopLoading();
        swal.close();
      }
    });
});