可以试试这个插件，试过没问题

(function($)
{
    // check for native placeholder support. Borrowed from Modernizr.
    var placeholderSupport = !!( 'placeholder' in document.createElement('input') );

    $.fn.placeholder = function(text)
    {
        // if placeholder is supported natively, exit
        if (placeholderSupport) return this;

        // else:
        return this.each(function()
        {
            var $this = $(this);

            text = text || $this.attr('placeholder');

            // if it's not a input element, exit
            if ( this.tagName !== "INPUT" )
            {
                $.error('jquery.placeholder only works on "input" elements. Does not support "' + this.tagName.toLowerCase() + '" elements.');
                return;
            }

            // If placeholder has already been applied, exit
            if ( $this.data('jquery-placeholder')  ) return;

            // If not, let's mark it now as having placeholder applied
            $this.data('jquery-placeholder', true);

            // if its value is empty, let's add the placeholder text
            if ( $this.val() === '' )
            {
                $this.val( text ).addClass('placeholder');
            }

            // Now, let's setup the focus & blur events, to add and remove the text & the class, respectively.
            $this.focus(function()
            {
                var $this = $(this);
                if ( $this.val() === text && $this.hasClass('placeholder') )
                {
                    $this.val('').removeClass('placeholder');
                }
            })
            .blur(function()
            {
                var $this = $(this);
                if ( $this.val() === '')
                {
                    $this.val( text ).addClass('placeholder');
                }
            });

        });
    };

    // Now, before we leave, let's just make sure that these placeholder values never get submitted
    placeholderSupport || $(document).delegate('form', 'submit', function()
    {
        $(this)
            .find('.placeholder')
            .val('').removeClass('placeholder');
    });

})(jQuery);

$('input[placeholder]').placeholder();

fiddle地址: http://jsfiddle.net/vNkPD

var abc='xyz';
var xyz=777;
alert(window[abc]);

//写闭包中就这样
(function(){
    abc='xyz';
    xyz=777;
    alert(window[abc]);
})();

可以试试 Flexbox

.box1 {
  background: black;
  height: 400px;
  width: 400px;
  margin: auto;
  display:flex;
  align-items: center;
  justify-content: center;
}

.box2 {
  height:300px;
  width:300px;
  background: red;
  margin: auto;
}

<div class="box1">
  <div class="box2">
  </div>
</div>

这个主要取决于后台的验证方式，如果是用token的方式, 那你就要在请求头部设置token, 也可以通过session ,cookie 进行验证