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<div class="wrapper">
    <span class="span-1">Lorem ipsum dolor sit amet</span>
</div>
<br>
<div class="wrapper">
    <span class="span-2">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Molestias omnis aliquid fugit necessitatibus rerum beatae alias dolore. Deleniti labore illo!</span>
</div>
<!-- 
    span标签会产生inline-level boxes，这里用两个span作为对比，
    span-1行高为1，即行高和字体大小相等，为20px
    span-2行高设定为30px，大于字体大小20px
    外层div的高度为默认值（auto），从而div的高度将等于内层span的高度
 -->

div{
    background-color:#0e51a7;/*div用作参照物*/
}
span {
    background-color: #ff9e00;
    font-size: 20px;
}
.span-1{
    line-height: 1;
}
.span-2{
    line-height: 30px;
}