LayoutTests/fast/css/many-rules.html

<body>
<div id=info></div>
<div id=test></div>
<script>
function makeSheet(index) {
    let sheet;
    sheet = `#test:after { content: "Matched rule from sheet ${index}" }`;
    for (let i = 1; i < 32768; ++i)
        sheet += ".dummy-rule { color: red }";
    return sheet;
}
function makeSheets(count) {
    for (let i = 0; i < count; ++i) {
        const style = document.createElement("style");
        style.textContent = makeSheet(i + 1);
        document.head.appendChild(style);
    }

    info.textContent = `Expecting match from sheet ${count}`;
}
makeSheets(20);
</script>
</body>