W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
* {box-sizing: border-box}

/* Set height of body and the document to 100% */
body, html {
  height: 100%;
  margin: 0;
  font-family: Arial;
}

/* Style tab links */
.tablink {
  background-color: #555;
  color: white;
  float: left;
  border: none;
  outline: none;
  cursor: pointer;
  padding: 14px 16px;
  font-size: 17px;
  width: 25%;
}

.tablink:hover {
  background-color: #777;
}

/* Style the tab content (and add height:100% for full page content) */
.tabcontent {
  color: white;
  display: none;
  padding: 100px 20px;
  height: 100%;
}

#Home {background-color: red;}
#News {background-color: green;}
#Contact {background-color: blue;}
#About {background-color: orange;}
</style>
</head>
<body>

<button class="tablink" onclick="openPage('Home', this, 'red')">Home</button>
<button class="tablink" onclick="openPage('News', this, 'green')" id="defaultOpen">News</button>
<button class="tablink" onclick="openPage('Contact', this, 'blue')">Contact</button>
<button class="tablink" onclick="openPage('About', this, 'orange')">About</button>

<div id="Home" class="tabcontent">
  <h3>Home</h3>
  <p>Home is where the heart is..</p>
</div>

<div id="Contact" class="tabcontent">
  <h3>Contact</h3>
  <p>Get in touch, or swing by for a cup of coffee.</p>
</div>

<div id="About" class="tabcontent">
  <h3>About</h3>
  <p>Who we are and what we do.</p>
</div>

// Get the element with id="defaultOpen" and click on it
document.getElementById("defaultOpen").click();
</script>
   
</body>

</html>

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<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width, initial-scale=1"><style>* {box-sizing: border-box}​/* Set height of body and the document to 100% */body, html {  height: 100%;  margin: 0;  font-family: Arial;}​/* Style tab links */.tablink {  background-color: #555;  color: white;  float: left;  border: none;  outline: none;  cursor: pointer;  padding: 14px 16px;  font-size: 17px;  width: 25%;}​.tablink:hover {  background-color: #777;}​/* Style the tab content (and add height:100% for full page content) */.tabcontent {  color: white;  display: none;  padding: 100px 20px;  height: 100%;}​#Home {background-color: red;}#News {background-color: green;}#Contact {background-color: blue;}#About {background-color: orange;}</style></head><body>​