各浏览器下placeholder的设置方法

::-webkit-input-placeholder { /* WebKit browsers */
color:#999;
}
:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
color:#999;
}
::-moz-placeholder { /* Mozilla Firefox 19+ */
color:#999;
}
:-ms-input-placeholder { /* Internet Explorer 10+ */
color:#999;
}