Color cycling works with paletted images. CSS blending modes work in RGB with a limited set of blend functions. The closest thing to color cycling is "filter: hue-rotate(...)", but you can't limit the colors to a particular subset of the hue wheel, so it's gonna be useless.

Right, but the colors within the SVG are what is cycling. It's not a rasterized image, so you could create a "pixel map". I'm not saying it's not convoluted as hell, but the effect [w|c]ould look paletted

At that point what's the purpose of having a mask image + blending at all?

because you can now overlay onto other RGB rasterized images

The SVG is just the spectral map. We're using its native possibilites to pull off some other crazy wild hare of an idea to do things that other image compositing programs do, but in a browser. I'm way way outside a box here, but that's where I prefer to play. This is one of those things where I'm taking an idea that someone developed with a specific idea, but I'm jerking the wheel hard left to suggest something zanny just because it might be interesting and cool. Following directions and creator's intent is boring ;-)

You can also overlay animated <canvas> elements.

I don’t really get what you’re saying, but ok.