<html><body><h1>select required 属性</h1><p>required 属性属性规定在提交表单之前，用户必须选择一个值：</p><form action="/demo/action_page.php"> <label for="cars">请选择一个汽车品牌：</label> <select name="cars" id="cars" required> <option value="">None</option> <option value="audi">奥迪</option> <option value="byd">比亚迪</option> <option value="geely">吉利</option> <option value="volvo">沃尔沃</option> </select> <br><br> <input type="submit" value="提交"></form></body></html>