The MathJax exploit for CSS injection was fun while it lasted.

$\unicode[monospace; pointer-events: none; z-index: -1; position: fixed; top: 0; left: 0; height: 100%; width: 100%; opacity: 0.5; background-size: cover; background-position: bottom center, 50%, 50%; background-image: url("data:image/svg+xml;base64,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");]{x1F605}$