A brief discussion on the fun of :focus-within in CSS

<div class="ctn">
   
  <div class="xPassword">
    <input type="password" placeholder="Please enter your password" class="input">
    <img src="http://suo.im/5UnnjN" alt=""> 
  </div>
    
  <img src="http://r6m.cn/csAC" alt=""> 
    
</div>

.ctn {
     position: relative;
    width: 318px;
    margin: 100px auto;
    height: 370px;
    padding: 20px;
    box-sizing: border-box;
    background: #fff;
    z-index: 10;
    box-shadow: 0 0 15px #cfcfcf;
    
}
.ctn h2 {
  font-size: 20px;
  font-weight: bold;
  margin-bottom: 30px;
}
.ctn input {
  padding: 10px;
  width: 100%;
  border: 1px solid #e9e9e9;
  border-radius: 2px;
  outline: none;
  box-sizing: border-box;
  font-size: 16px;
}
img {
    position: absolute;
    top: -23%;
    left: 50%;
    width: 80px;
    height: auto;
    transform: translate(-50%, 0);
}

.xPassword img {
  display: none;
  width: 103px;
  height: auto;
  top: -26%;
}

//The above are worthless and can be ignored. The following two are the key points. xPassword:focus-within ~ img {
  display: none;
}

.xPassword:focus-within img {
  display: block;
}