既能实现下拉又能实现输入的文本框

<div style="position:relative;"> 
<span style="margin-left:100px;width:18px;overflow:hidden;"> 
<select style="width:118px;margin-left:-100px" onchange="this.parentNode.nextSibling.value=this.value"> 
<option value="Ajax">Ajax</option> 
<option value="XML">XML</option> 
<option value="xmlHTTP">xmlHTTP</option> 
<option value="CSS">CSS</option> 
</select></span><input name="tag" style="width:100px;position:absolute;left:0px;height:21px;" value="输入或选择" onmouseover="this.select()"> 
</div>