Block elements Right / Left

html part

<div class="father">
     <div class="son"></div>
</div>

css section

 .father {
    width: 300px;
    background: green;
  }

  .son {
    width: 200px;
    height: 200px;
    margin-right: auto;   //左对齐
    background: red;
  }

 .father {
    width: 300px;
    background: green;
  }

  .son {
    width: 200px;
    height: 200px;
    margin-left: auto;   ///右对齐
    background: red;
  }