Noob knockout practice

HTML

<div class="content">
  <p>
    First:
    <input data-bind="value:firstName"></input>
  </p>
  <p>
    Last:
    <input data-bind="value:lastName"></input>
  </p>
  <p>
    Full: &nbsp;&nbsp;<span data-bind="text:fullName"></span>
  </p>


</div>

CSS

div {
  background-color: #f20056;
  padding: 2em 2em 2em 2em;
  opacity: 0.95;
  color: white;
}

JavaScript

var ViewModel = function(first, last) {
  var self = this;
  self.firstName = ko.observable(first);
  self.lastName = ko.observable(last);

  self.fullName = ko.computed(function() {
    return self.firstName() + " " + self.lastName();
  });

};

ko.applyBindings(new ViewModel('', ''));