可输入的下拉框

<select name="selectmenu" 
style="position:absolute;top:0px; width: 110px;clip: rect(0 110 20 90) " 
onChange="document.getElementById('dmtxt').value=this.options[this.selectedIndex].value">
<option value="">选择邮箱</option>
<option value="sina.com">sina.com</option>
<option value="sohu.com">sohu.com</option>
</select>
<input name="dmtxt" id="dmtxt" type="text"
style="position:absolute;top:0px; width:95px;">

这个比较简单，还有比较麻烦的用JS实现