title: AngularJS - difference between pristine/dirty and touched/untouched
body: <p><a href="https://docs.angularjs.org/guide/forms" rel="noreferrer">AngularJS Developer Guide - Forms</a> lists many styles and directives regarding forms and fields. For each one, a CSS class:</p>
  <pre><code>ng-valid
  ng-invalid
  ng-pristine
  ng-dirty
  ng-touched
  ng-untouched
  </code></pre>
  <p>What's the difference between <code>pristine/dirty</code>, and <code>touched/untouched</code>?</p>