html 弹出信息原理是怎样的?半路出家搞前端求大神指点

Question

Answer 1

<style type="text/css">
.box {
  width: 200px;
  height: 200px;
  background: #eee;
  border: 1px solid transparent;
}
.box .extra {
  display: none
}
.box:hover {
  border-color: red;
}
.box:hover .extra {
  display: block
}
</style>
<div class="box">
  <div class="info">info</div>
  <div class="extra">extra</div>
</div>

Answer 2

前端学习基本技能，看看源码，拿过来就用