.phcolor{?
???color:#999;
}

<input?type="text"?id="uname"?name="uname"?placeholder="請輸入用戶名"/>

<script>
?$(function(){??
??//判斷瀏覽器是否支持placeholder屬性
??supportPlaceholder='placeholder'in?document.createElement('input'),
??placeholder=function(input){
????var?text?=?input.attr('placeholder'),
????defaultValue?=?input.defaultValue;
????if(!defaultValue){
??????input.val(text).addClass("phcolor");
????}
????input.focus(function(){
??????if(input.val()?==?text){
????????$(this).val("");
??????}
????});
????input.blur(function(){
??????if(input.val()?==?""){
????????$(this).val(text).addClass("phcolor");
??????}
????});
????//輸入的字符不為灰色
????input.keydown(function(){
??????$(this).removeClass("phcolor");
????});
??};
??//當瀏覽器不支持placeholder屬性時，調用placeholder函數
??if(!supportPlaceholder){
????$('input').each(function(){
??????text?=?$(this).attr("placeholder");
??????if($(this).attr("type")?==?"text"){
????????placeholder($(this));
??????}
????});
??}
});
</script>

ie10會有點小問題，其他ie沒問題

解決IE8的兼容性問題
<script?type="text/javascript">??
??if(?!('placeholder'?in?document.createElement('input'))?){???
????$('input[placeholder],textarea[placeholder]').each(function(){???
??????var?that?=?$(this),???
??????text=?that.attr('placeholder');???
??????if(that.val()===""){???
????????that.val(text).addClass('placeholder');???
??????}???
??????that.focus(function(){???
????????if(that.val()===text){???
??????????that.val("").removeClass('placeholder');???
????????}???
??????})???
??????.blur(function(){???
????????if(that.val()===""){???
??????????that.val(text).addClass('placeholder');???
????????}???
??????})???
??????.closest('form').submit(function(){???
????????if(that.val()?===?text){???
??????????that.val('');???
????????}???
??????});???
????});???
??}??
</script>