为什么我写的不会跟随？哪里不对？
<style>
.content { width: 60%; height: 2000px; background: #999; margin: 0 auto;}
.h0 { height: 0;}
.ovh { overflow: hidden;}
.tr { text-align: right;}
.abs { position: absolute; width: 20px; height: 20px; background: #F00;}
.ml10 { margin-left: 10px;}
.mt30 { margin-top: 30px;}
</style>
<body>
<div class="content">
<div class="h0 ovh tr">
<span class="abs ml10 mt30"></span>
</div>
</div>
</body>
