CSS Border不会包含浮动项目

.parent {
  border: 10px solid transparent;
  background-color: #f0f0f0;
}

.parent {
  position: relative;
}

.parent::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  border: 10px solid #f0f0f0;
  z-index: -1;
}

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

.parent {
  position: relative;
}