<input value="请输入搜索内容" onfocus="javascript:this.value=''" type="text" id="ipt1" />
<input value="提交" type="Submit" id="btn1" onclick="checkform()" />
<script type="text/javascript">
function checkform(){
if(document.getElementById('ipt1').value.length==0){
alert('输入为空！');
document.getElementById('ipt1').focus();
return false;
}
if(document.getElementById('ipt1').value=="请输入搜索内容"){
alert('输入为空！');
document.getElementById('ipt1').focus();
return false;
}
}
</script>
