chromium/third_party/blink/web_tests/fast/forms/select/select-selected.html

<select>
    <optgroup label="group A">
        <option>option 1</option>
    </optgroup>
    <optgroup label="group B">
        <option>option 2</option>
    </optgroup>
    <option selected="selected">should see this option selected</option>
    <option>should NOT see this option selected</option>
    <option>should NOT see this option selected, either</option>
</select>