SOURCE

<!-- 如果试图触发div标签绑定的onfocus事件和onblur事件，需要为该标签添加tabindex属性  -->
<div class="a" tabindex="0">
  <div class="b"></div>
</div>
<div class="c"></div>

.a{
  outline: none;
}
.b{
  width: 50px;
  height: 50px;
  background: red;
}
.c{
  width: 100px;
  height: 100px;
  background: blue;
}
.a:focus + .c{
  background: yellow;
}