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chromium/third_party/blink/web_tests/fast/css-generated-content/attr-minus.html 

<!DOCTYPE html>
<style>
    div::before { content: attr(-foo); }
</style>
<div foo="FAIL">PASS</div>
<div -foo="PASS"></div>
<!-- Note that the selector is valid but strictly speaking it should not match since -foo attribute should have been dropped. -->