<html><head><script>function validateForm() { var x = document.forms["myForm"]["fname"].value; if (x == "" || x == null) { alert("Name must be filled out"); return false; }}</script></head><body><h1>JavaScript 验证空输入框</h1><p>请尝试在不输入任何文本的情况下提交表单。</p><form name="myForm" action="/demo/action_page.php" onsubmit="return validateForm()" method="post" required> Name: <input type="text" name="fname"> <input type="submit" value="Submit"></form></body></html>