JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
</head>
<body>
    <h1>Test</h1>
    <hr>
    <p class="test">a</p>
    <button type="button" onclick='myFunction();'>Click Me!</button>
</body>
</html>

JavaScript

function myFunction() {
    var ps = document.getElementsByClassName('.test');
    for (var p of ps) {
        p = p.replace('a', '<span style="color:blue;">b</span>');
		}
    alert("Working?");
    return null;
}