Why isn't it being assembled as HTML that can be viewed in document inspector or via viewing source? Ht HTML that extensions inspect shows up. If it is assembled to be rendered, why isn't it available anywhere else in the browser to view?
Blank rickroll site from 3 years ago using same technique: <a href="https://github.com/PwnFunction/Blank-Rick-Roll">https://github.com/PwnFunction/Blank-Rick-Roll</a>
TL; DR: It uses the HTTP link header [1] in a way that only works on some browsers:<p><pre><code> link <data:text/css;charset=utf-8;base64,KiB7CiAgICBiYWNrZ3JvdW5kLWNvbG9yOiBjeWFuOwp9Cgpib2R5OjphZnRlciB7CiAgY29udGVudDogIkhpISBUaGlzIGlzIHNlcnZlZCBlbnRpcmVseSBpbiB0aGUgSFRUUCBoZWFkZXJzISBQcmV0dHkgY29vbCwgaHVoPyI7CiAgZm9udC1zaXplOiAyNHB4OwogIGZvbnQtZmFtaWx5OiBjdXJzaXZlOwogIHRleHQtc2hhZG93OiAycHggMnB4IDJweCBwaW5rOwp9Cg==>; rel="stylesheet"
</code></pre>
which is, when base64-decoded:<p><pre><code> * {
background-color: cyan;
}
body::after {
content: "Hi! This is served entirely in the HTTP headers! Pretty cool, huh?";
font-size: 24px;
font-family: cursive;
text-shadow: 2px 2px 2px pink;
}
</code></pre>
[1] <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Li...</a>