IE10 以下版本完美支持 placeholder 特性

$(function () {
//浏览器不支持 placeholder 时才执行
if (!('placeholder' in document.createElement('input'))) {
$('[placeholder]').each(function () {
var $tag = $(this); //当前 input
var $copy = $tag.clone();   //当前 input 的复制
            if ($copy.val() == "") {
                $copy.css("color", "#999");
                $copy.val($copy.attr('placeholder'));
            }
$copy.focus(function () {
if (this.value == $copy.attr('placeholder')) {
this.value = '';
this.style.color = '#000';
}
});
$copy.blur(function () {
if (this.value=="") {
this.value = $copy.attr('placeholder');
$tag.val("");
this.style.color = '#999';
} else {
$tag.val(this.value);
}
});
$tag.hide().after($copy.show());    //当前 input 隐藏 ，具有 placeholder 功能js的input显示
});
}
});