点击运行
<!DOCTYPE html> <html> <body> <form action="/zy/demo/demo_form.php"> <select required> <option value="volvo">Volvo</option> <option value="saab">Saab</option> <option value="mercedes">Mercedes</option> <option value="audi">Audi</option> </select> <input type="submit"> </form> <p><b>注释：</b>所有主流浏览器都不支持 required 属性。</p> </body> </html>
运行结果
