<html><body><form id="frm1" action="/action_page.php"> First name: <input type="text" name="fname" value="Donald"><br> Last name: <input type="text" name="lname" value="Duck"><br><br> <input type="submit" value="提交"></form><p>单击“试一试”以显示表单的元素数：</p><button onclick="myFunction()">试一试</button><p id="demo"></p><script>function myFunction() { var x = document.getElementById("frm1").length; document.getElementById("demo").innerHTML = x;}</script></body></html>