Edit in JSFiddle

<p>When an option's index matches the index of it's optgroup among sibling optgroups, there is a failure. For example: optgroup 0 and option 0, or optgroup 2 and option 2.</p>

<form>
    <select required>
        <optgroup label="Group 0">
            <option>Option 0</option>
            <option>Option 1</option>
        </optgroup>
        <optgroup label="Group 1">
            <option>Option 0</option>
            <option>Option 1</option>
        </optgroup>
        <optgroup label="Group 2">
            <option>Option 0</option>
            <option>Option 1</option>
            <option>Option 2</option>
        </optgroup>
        <optgroup label="Group 3">
            <option>Option 0</option>
            <option>Option 1</option>
            <option>Option 2</option>
            <option>Option 3</option>
        </optgroup>
        <optgroup label="Group 4">
            <option>Option 0</option>
            <option>Option 1</option>
            <option>Option 2</option>
            <option>Option 3</option>
            <option>Option 4</option>
        </optgroup>
    </select>
    <input type="submit" />
</form>