* {
    margin: 0;
    padding: 0;
    /* 內减模式 */
    box-sizing: border-box;
}

li {
    list-style: none;
}
a {
    text-decoration: none;
}

.clearfix:before,.clearfix:after {
    content:"";
    display:table; 
  }
  .clearfix:after {
    clear:both;
  }