​x
 
<!DOCTYPE html>
<html>
<body>
​
<form action="/demo/html/action_page.php" method="get">
  Name: <input type="text" id="myInput" name="fname" required>
  <input type="submit" value="Submit">
</form>
​
<p>如果您点击提交，而不填写文本字段，将出现警告消息。</p>
​
<p><b>注释：</b>Internet Explorer 9 及更早版本或 Safari 不支持 oninvalid 事件。</p>
​
<script>
document.getElementById("myInput").addEventListener("invalid", myFunction);
​
function myFunction() {
  alert("You must fill out the form!");
}
</script>
​
</body>
</html>
​