How to center image in div

<p class="title">
        <p class="flag"></p>
        <p class="content">
            <img src="img_p1_title.png">
        </p>
    </p>

.title {
   width: 100%;
    font-size: 0;  
    height: 10%;
}
.title .content img {
    width: 35%;
}
/*--主要的--*/.content{
    display: inline-block;
    vertical-align: middle;
  }
.flag{
    display: inline-block;
    vertical-align: middle;
    height: 100%;
    width: 0;
  }

<p class="title">
<img src="img_p1_title.png">
</p>

<span style="color: #000000;">.title {  <span style="color: #ff0000;">display: flex;
  just<a href="//m.sbmmt.com/wiki/109.html" target="_blank">if</a>y-content: center;
  align-items: center; </span> }
.title img {
  width: 35%;
}<br/>//该方法有些旧系统不支持</span>

<p class="title">
<span>第三种方法</span>
</p>

.title {
   height: 15%;
   font-size: 18px;   position: relative;
}
.title span {    
        position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

<p class="title">
 <span>第四种方法</span>
</p>

.title {
    width: 200px;
    height: 200px;    
    vertical-align: middle;
    display: table-cell;
    text-align: center;
 }