<html>
    <head>
        <script>
            function sendMessage() { alert(getComputedStyle(document.querySelector(".hidden")).display == "none" ? "content blockers enabled" :"content blockers disabled"); }
        </script>
    </head>
    <body onload="sendMessage()">
        <p class="hidden">
            This text exists to determine if content blockers are enabled.
            If they're enabled, the computed style of this text will be display:none.
        </p>
    </body>
</html>