XHTML标记验证错误

Question

I'm trying to validate my XHTML but I'm getting two errors firstly "end tag for "li" omitted, but OMITTAG NO was specified" and " document type does not allow element "ul" here; assuming missing "li" start-tag."

How do I solve this whilst still keeping the bottom two links a sub list of the first one? Thanks

<ul>
<li><a href="#div_into">Introduction</a></li>
<li ><a href="#div_hist">History</a></li>
<li><a href="#div_national">National Flags</a></li>
<li><a href="#div_signalFlags">International Maritime Signal Flags</a></li>
    <ul>
        <li><a href="#div_letters">International Maritime Signal: Letters</a></li>
        <li><a href="#div_numbers">International Maritime Signal: Numbers</a></li>
    </ul>

Answer 1

<ul>
    <li><a href="#div_into">Introduction</a></li>
    <li><a href="#div_hist">History</a></li>
    <li><a href="#div_national">National Flags</a></li>
    <li><a href="#div_signalFlags">International Maritime Signal Flags</a>
        <ul>
            <li><a href="#div_letters">International Maritime Signal: Letters</a></li>
            <li><a href="#div_numbers">International Maritime Signal: Numbers</a></li>
        </ul>
    </li>
</ul>

Only a <li> element can be a child of a <ul> . You have a starting list after the International Maritime Signal Flags item which should be within that item, not after it.