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chromium/third_party/blink/web_tests/fast/forms/color-scheme/password/password-with-reveal-button.html 

<!DOCTYPE html>
<meta name="color-scheme" content="light dark">

<input type="password" id="passwd">

<script>
    // This verifies the reveal-password button appears after a character is entered.
    document.getElementById("passwd").focus();
    eventSender.keyDown('a');
</script>