<html><head><script>function validateForm() { let x = document.forms["myForm"]["fname"].value; if (x == "") { alert("Name must be filled out"); return false; }}</script></head><body><h1>JavaScript 验证</h1><form name="myForm" action="/demo/html/action_page.php" onsubmit="return validateForm()" method="post"> Name: <input type="text" name="fname"> <input type="submit" value="Submit"></form></body></html>