A silly little web dev joke. If you use a browser that isn't Blink-based, you will be safe. If you do use Blink...
In addition to setting the angel image depending on the rendering engine, the attacking angel version plays a CSS animation that fakes slow loading of the image to jump-scare the viewer when the face of the angel suddenly appears all at once. Normally, you would animate the cover's height using a transform, but in this case, I animate its height to keep the cover at pixel boundaries and avoid spoiling the illusion.
