Hi makenzi.exc,
Apply italics method of string like in the following example.
Refer below example.
HTML
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
body { font-family: Arial; font-size: 10pt; }
</style>
</head>
<body>
<div id="dvContent" align="center">
Welcome to ASPSnippets
</div>
<script type="text/javascript">
window.onload = function () {
var dvContent= document.getElementById("dvContent");
// Apply italics method.
let italicString = dvContent.innerHTML.italics();
// Display italic string.
dvContent.innerHTML = italicString;
};
</script>
</body>
</html>
Demo
Screenshot

Downloads
Download Sample
Note: String italics is deprecated in JavaScript.
So avoid using it. Instead that use the i tag.
<i>Welcome to ASPSnippets</i>