Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<head>
<style>
.mystyle {
    width: 300px;
    height: 50px;
    background-color: coral;
    color: white;
    font-size: 25px;
}
</style>
</head>
<body>

<h2>Cross-browser solution for classList.add()</h2>
<p>The classList property is not supported in Internet Explorer 9 and earlier.</p>
<p>In this example, we check if the browser supports the classList.add() method. If not, use the className property instead to achieve the same result (for IE9 and earlier).</p>

<p>Click the button to add the class "mystyle" to the DIV element.</p>

<button onclick="myFunction()">Try it</button>

<div id="myDIV">
I am a DIV element
</div>

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

<script>
function myFunction() {
    var x = document.getElementById("myDIV");

    if (x.classList) { 
        x.classList.add("mystyle");
    } else {
        x.className = "mystyle";
    }
}
</script>

</body>
</html>