现场演示：我的 HTML 文档

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>Writing into an HTML Element with JavaScript</title>
</head>
<body>
    <p id="greet"></p>
    <p id="result"></p>
    
    <script>
    // Writing text string inside an element
    document.getElementById("greet").innerHTML = "Hello World!";
    
    // Writing a variable value inside an element
    var x = 10;
    var y = 20;
    var sum = x + y;
    document.getElementById("result").innerHTML = sum;
    </script>
</body>
</html>