title: Show hidden element
body: <p>I have <code>div</code> and hidden <code>span</code> on it. I want to show it by button clicking. My code:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
    &lt;head&gt;
      &lt;meta charset="utf-8" /&gt;
      &lt;title&gt;register page&lt;/title&gt;
      &lt;link rel="stylesheet" href="/static/css/bootstrap.min.css"&gt;
      &lt;script type="text/javascript" src="/static/js/test.js"&gt;&lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
      &lt;form id="register-form"&gt;           
          &lt;label&gt;Username&lt;span id="register-username-label" class="label label-important" style="visibility: hidden;"&gt;Username can't be empty&lt;/span&gt;&lt;/label&gt;
          &lt;input id="register-username-input" type="text" placeholder="Type username..."/&gt;
          &lt;button id="register-submit" onclick="show_element('register-username-label')" class="btn"&gt;Register&lt;/button&gt;
          &lt;/form&gt;
      &lt;/div&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>And my js file:</p>
  
  <pre><code>function show_element (id) {
      document.getElementById(id).style.visibility = 'visible';
  }
  </code></pre>
  
  <p>When i button clicked span does not show. Why? It shows very fast and than hidden again.</p>
  
  <p>Chrome developer tools console doesn't show errors too. </p>
  
  <p>Thank you.</p>