<select name="province" id="province" required="required"> 
    <optgroup label="Provinces">
         <option value="Alberta">Alberta</option>
         <option value="British Colombia">British Colombia</option>
         <option value="Manitoba">Manitoba</option>
         <option value="New Brunswick">New Brunswick</option>
         <option value="Newfoundland and Labrador">Newfoundland and Labrador</option>
         <option value="Nova Scotia">Nova Scotia</option>
         <option value="Ontario" selected="selected">Ontario</option>
         <option value="Prince Edward Island">Prince Edward Island</option>
         <option value="Quebec">Quebec</option>
         <option value="Saskatchewan">Saskatchewan</option>
    </optgroup>
         <optgroup label="Territories">
         <option value="Northwest Territories">Northwest Territories</option>
         <option value="Nunavut">Nunavut</option>
         <option value="Yukon">Yukon</option>
    </optgroup>
</select>

我需要具有与指南所要求的选项相同的值。此代码工作正常但不通过W3C验证：

A select element with a required attribute and without a multiple attribute,and whose size is 1,must have a child option element.

这个问题的解决方案是什么？