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chromium/third_party/blink/web_tests/fast/css-generated-content/quotes-lang-case-insensitive.html 

<!DOCTYPE html>

<style>
    :lang(CusTom) {
        quotes: "A" "B" "C" "D";
    }
</style>

<p>
    You should see the below line printed twice.
</p>

ACDB<br>
<q lang="CUSTOM"><q></q></q>