<html><body><form action="/demo/demo_form.asp"><select name="cars" 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>