Untitled

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>this</title>

        <style>
        .green-text {
            color: green;
        }

        .blue-text {
            color: blue;
        }
        </style>
    </head>

    <body>
        <h1 id="hello" class="ninja">Hello, ninja!</h1>

        <button id="blueButton">Change to blue</button>

        <script src="script.js"></script>
    </body>
</html>