<html><body><form action="/demo/htmldom/action_page.php"> 密码：<input type="password" id="myPsw" name="usr_psw"> <input type="submit"></form><p>单击“试一试”按钮可为密码字段设置 required 属性。</p><p><b>提示：</b>请在点击“试一试”按钮前后点击“提交”按钮，查看效果。</p><button onclick="myFunction()">试一试</button><p id="demo"></p><script>function myFunction() { document.getElementById("myPsw").required = true; document.getElementById("demo").innerHTML = "required 属性已设置。现在必须在提交表单之前填写密码字段。";}</script></body></html>