Tools/TestWebKitAPI/Tests/WebKitCocoa/nested-frames.html

<!DOCTYPE html>
<html>
<head>
<title>Outer Subframe</title>
<style>
iframe {
    width: 100px;
    height: 100px;
    border: 1px solid black;
}
</style>
</head>
<body>
    <p>The round pegs in the square holes.
        <iframe title="Inner Subframe"></iframe>
    </p>
    <script>
        addEventListener("load", () => {
            const subframe = document.querySelector("iframe");
            subframe.addEventListener("load", () => {
                parent.postMessage("subframeLoaded", "*");
            });
            subframe.srcdoc = `
                <head>
                    <title>Inner Subframe</title>
                </head>
                <body>
                    <a href='https://webkit.org'>The ones who see things differently.</a>
                </body>`;
        });
    </script>
</html>