​x
 
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<!DOCTYPE html>
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<title>My Example</title>
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<style>
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    .example {
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        font-family: Helvetica, Arial, sans-serif;
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        font-size: 4vw;
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    }
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</style>
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​
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<p class="example">Some random text to demonstrate the effect.</p>
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​
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<p>Helvetica will be used if the user's system has it, otherwise Arial will be used. If Arial isn't available, then the system's default sans-serif font will be used.</p>