​x
 
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<!DOCTYPE html>
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<title>My Example</title>
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​
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<style>
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  nav {
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    width: 20%;
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    margin-right: 1em;
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    float: left;
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    background: gold;
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  }
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  article {
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    width: calc(80% - 1em);
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    float: right;
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    background: orange;
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  }
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  article, nav {
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    color: white;
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    padding: 30px;
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    box-sizing: border-box;
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  }
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</style>
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​
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<nav>
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  <h1>Nav</h1>
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</nav>
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<article>
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  <h1>Article</h1>
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</article>