<html><body><h1>optgroup 元素</h1><p>optgroup 标签用于对下拉列表中的相关选项进行分组：</p><form action="/action_page.php"> <label for="cars">选择一辆车：</label> <select name="cars" id="cars"> <optgroup label="Swedish Cars"> <option value="volvo">Volvo</option> <option value="saab">Saab</option> </optgroup> <optgroup label="German Cars"> <option value="mercedes">Mercedes</option> <option value="audi">Audi</option> </optgroup> </select> <br><br> <input type="submit" value="提交"></form> </body></html>