Codebase Browser
chromium
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chromium/third_party/blink/web_tests/fast/css-generated-content/001.html 

<html>
<head>
<style>
a:before { content: "Link: "; }
a:before { display: none; }
</style>
</head>
<body>
<a href="foo.html">You should not see anything but this link.</a>
</body>