Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>Hello World!</p>

<p class="intro">The DOM is very useful.</p>
<p class="intro">This example demonstrates the <b>getElementsByClassName</b> method.</p>

<p id="demo"></p>

<script>
var x = document.getElementsByClassName("intro");
document.getElementById("demo").innerHTML = 
'The first paragraph (index 0) with class="intro": ' + x[0].innerHTML;
</script>

</body>
</html>