CSS による div コンテンツの垂直中央揃え ケースサマリー

p { height:30px; line-height:30px; width:100px; overflow:hidden; }

p { padding:20px 0; }

<div id="wrapper">
    <div id="cell">
        <p>Test vertical centering effectTest vertical centering effect</p>
        <p>Testing vertical centering effectTesting vertical centering effect</p>
    </div>
</div>

#wrapper {display:table;width:300px;height:300px;background:#000;margin:0 auto;color:red;}
#cell{display:table-cell; vertical-align:middle;}

.center-vertical{
  position: relative;
  top:50%;
  transform:translateY(-50%);
}.center-horizontal{
  position: relative;
  left:50%;
  transform:translateX(-50%); 
}

<div class="center">
  <div class="text">
    <p>I am multi-line text</p>
    <p>I am multi-line text</p>
    <p>I am multi-line text</p>
  </div>
</div>

.center {
  width: 300px;
  height: 200px;
  padding: 10px;
  border: 1px solid #ccc;
  background:#000;
  color:#fff;
  margin: 20px auto;


  display: -webkit-box;
  -webkit-box-orient: horizontal;
  -webkit-box-pack: center;
  -webkit-box-align: center;
  
  display: -moz-box;
  -moz-box-orient: horizontal;
  -moz-box-pack: center;
  -moz-box-align: center;
  
  display: -o-box;
  -o-box-orient: horizontal;
  -o-box-pack: center;
  -o-box-align: center;
  
  display: -ms-box;
  -ms-box-orient: horizontal;
  -ms-box-pack: center;
  -ms-box-align: center;
  
  display: box;
  box-orient: horizontal;
  box-pack: center;
  box-align: center;
}

<div class="flex">
    <div>
       <p>I am multi-line textI am multi-line textI am multi-line textI am multi-line textI am multi-line text</p>
      <p> I am multi-line text I am multi-line text I am multi-line text I am multi-line text I am multi-line text </p>
    </div>
</div>

.flex{
    /*flex layout*/
    display: flex;
    /*centering vertically*/
    align-items: center;
    /*center horizontally*/
    justify-content: center;
    
    text-align: justify;
    width:200px;
    height:200px;
    background: #000;
    margin:0 auto;
    color:#fff;
}