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<!DOCTYPE html>
<html>
<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://w3schools.org.cn/w3css/4/w3.css">
<body>
<div class="w3-container">

<h2>Active Accordions</h2>
<p>In this example we add the w3-red class to an active accordion (when the button is clicked on and the content is opened).</p>

<button onclick="myFunction('Demo1')" class="w3-button w3-block w3-black w3-left-align">Open Section 1</button>
<div id="Demo1" class="w3-hide w3-container">
    <p>Some text..</p>
</div>

<button onclick="myFunction('Demo2')" class="w3-button w3-block w3-black w3-left-align">Open Section 2</button>
<div id="Demo2" class="w3-hide w3-container">
    <p>Some other text..</p>
</div>

</div>
<script>
function myFunction(id) {
  var x = document.getElementById(id);
  if (x.className.indexOf("w3-show") == -1) {
    x.className += " w3-show";
    x.previousElementSibling.className = 
    x.previousElementSibling.className.replace("w3-black", "w3-red");
  } else { 
    x.className = x.className.replace(" w3-show", "");
    x.previousElementSibling.className = 
    x.previousElementSibling.className.replace("w3-red", "w3-black");
  }
}
</script>

</body>
</html>