SOURCE

<div id="father">
  <div id="abc">
    <span id="firstChild">我的居中内容很长很长</span>
    <span id="secondChild">这是返回按钮</span>
  </div>
</div>

#father {
    display: flex;
    width: 200px;
    height: 40px;
    background: gray;
    position: relative;
    text-align: center;
}

#abc {
    display: inline-block;

}

#firstChild {
    background: green;
    /*
    position: absolute;
    margin-left: 50%;
    transform: translate(-50%, 0);
    */
}

#secondChild {
    background: red;
    float: right;
}