<body>
  <div class="header">
    <span>里面的蚊子</span>
  </div>
</body>
.header {
  width: 900px;
  height: 100px;
  background-color: red;

  margin: 1px auto;
  /* 让行内元素或者行内块居中 这样才是起作用的 */
  text-align: center;
}
/* 这个是不起作用的,并不能让行内元素或者行内块居中,需要在父级添加 */
span {
  text-align: center;
}

External CSS

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External JavaScript

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