title: Add validation to HTML5 required fields
body: <p>I am using HTML5's <code>required</code> attribute on my input elements and select boxes and PHP for validation.</p>
  
  <p>How can I show an alert if the <code>required</code> fields are not filled in? I tried using <code>onsubmit()</code> but the form is processed anyway and no alert is shown.</p>