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The Webkit implementation uses 2 layers of: looped keyframes, color transitions, multi-colorstop gradient backgrounds (which are then animated via background-position animation). And all that sits under two larger inset rgba box-shadows. [+ the gradients and shadows on the inner form itself]

That said, I've actually been pretty pleased performance-wise, I expected worse.

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Try it with 256px shadows. ;)

Well, when you're doing a minimum of 256000 calculations per page element, you should expect some slowness (assuming a radius 256 filter requires a 512x512 kernel to process). A less naive implementation of blurring blurs horizontally then vertically, for a rough estimate of 2512w*h calculations, which could still be a lot for a lot of page elements.

I guess what I'm trying to say is this: don't expect to render a ton of large-radius shadows in realtime unless you're rendering with a modern game engine on a fast GPU.

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