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<!doctype html>
<title>CSS test reference</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<style>
.test {
display: flex;
}
.test::before {
content: "";
display: block;
width: 100px;
height: 100px;
background-color: green;
}
</style>
<div class="test"></div>