title: show/hide fieldset in HTML is not working
body: <p>I try to hide an Fieldset initially and then show it with an button click.
  But its just show me the text for a second and then its gone again.</p>
  <p>I tried to do it also with visibility attribute but I got the same problem.</p>
  <p>Is there something I do wrong?</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>function myFunction() {
    var f1 = document.getElementById("f1");
    f1.style.display = "block";
  }</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#f1 {
    display: none;
  }
  
  #btn_submit {
    width: 23em;
    height: 4em;
    font-weight: bold;
    background-color: #C6C5C5;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;body style="background-color:#A0A0A0;"&gt;
  
    &lt;form action="" method="post"&gt;
      &lt;br&gt;
      &lt;div class="a"&gt;
        &lt;h1&gt;onfiguration / Configuration&lt;/h1&gt;&lt;br&gt;
  
        &lt;p&gt;
          &lt;font size="6"&gt;(9) Relais 1 / Relay 1&lt;/font&gt;&lt;br&gt;
        &lt;/p&gt;
        &lt;fieldset id="f1"&gt;&lt;br&gt;
          &lt;font size="4"&gt;9.1 Modus / Mode:&lt;/font&gt;&lt;br&gt;&lt;br&gt;
        &lt;/fieldset&gt;
        &lt;button onclick="myFunction()" id="myBtn"&gt;Read more&lt;/button&gt;
  
  &lt;/body&gt;</code></pre>
  </div>
  </div>
  </p>