<html>
<body>
<form action="form_action.asp">
名: <input type="text" id="myText" name="fname">
<input type="submit">
</form>
<p>点击 "点我" 按钮为文本域设置 required 属性。</p>
<p><strong>提示：</strong> 在点击“点我”按钮后点击 "Submit" 按钮查看效果。</p>
<button onclick="myFunction()">点我</button>
<p id="demo"></p>
<script>
function myFunction() {
document.getElementById("myText").required = true;
document.getElementById("demo").innerHTML = " required 属性已设置。文本域在表单提交时是必填字段。";
}
</script>
</body>
</html>