html表头rowpan无法正常工作

I have a table like this:

我有这样一张桌子:

<table border="1">
<thead>        
    <tr>
        <th colspan="2">Items</th>
        <th colspan="2">Type</th>
        <th colspan="4">Values</th>
        <th colspan="2">Date</th>
    </tr>        
    <tr>
        <th colspan="2"></th>
        <th colspan="2"></th>
        <th colspan="2">Before</th>
        <th colspan="2">After</th>
        <th colspan="2"></th>
    </tr>
</thead>
<tbody></tbody>
<table