csentry.js

$(document).ready(function() {

  // When an invalid input was submitted, the server
  // adds the "is-invalid" class to form fields to display
  // them in red with an error message
  // When starting to type again, we want to remove the error
  // message to not confuse the user
  $('input[type="text"]').keyup(function(event) {
    $(this).removeClass("is-invalid");
  });

});