div高度匹配表单（下拉列表）

Question

<div style="background:#fff; height:100% ">
    <form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post"  >
        <label for='selectboxvalue'>Sort by:</label>
    <select name="selectboxvalue" onChange="this.form.submit();" id="selectboxvalue"  >   
        <option value="datenew" <?php if ( $_POST["selectboxvalue"]=="datenew" ) echo "selected"; ?>>Newest</option>
        <option value="name" <?php if ( $_POST["selectboxvalue"]=="name" ) echo "selected"; ?>>Popular</option>
        <option value="pricelow" <?php if ( $_POST["selectboxvalue"]=="pricelow" ) echo "selected"; ?>> Price Lowest First</option>
        <option value="pricehigh" <?php if ( $_POST["selectboxvalue"]=="pricehigh" ) echo "selected"; ?>>Price Highest First</option>
        <option value="dateold" <?php if ( $_POST["selectboxvalue"]=="dateold" ) echo "selected"; ?>>Oldest</option>
    </select>  
    </form>
</div>

为什么div不包装表单？