Bootstrap Jumbotron类不渲染灰色突出显示

Question

我正在学习编码，而我在Bootstrap中的Jumbotron类根本没有呈现灰色高光。有人可以看看它并告诉我我做错了吗？

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<div class="Jumbotron">
  <h1>This is a Jumbotron</h1>
  <p>Bacon ipsum dolor amet pastrami shankle jerky tail, ground round hamburger sausage spare ribs strip steak burgdoggen pancetta</p>
  <button class="btn btn-success btn-lg">Hi there!</button>
</div>

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＆＃13;

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