解决placeholder不兼容问题

 <script>
    $(function(){
      if(!placeholderSupport()){   // 判断浏览器是否支持 placeholder
        $('[placeholder]').focus(function() {
          var input = $(this);
          if (input.val() == input.attr('placeholder')) {
            input.val('');
            input.removeClass('placeholder');
          }
        }).blur(function() {
          var input = $(this);
          if (input.val() == '' || input.val() == input.attr('placeholder')) {
            input.addClass('placeholder');
            input.val(input.attr('placeholder'));
          }
        }).blur();
      };
    })
    function placeholderSupport() {
      return 'placeholder' in document.createElement('input');
    }
  </script>