Fast Neural Style transfer is a variation of a "regular" Neural Style algorithm.

With regular Neural Style, you can do style transfer between any two arbitrary images. The disadvantage is it takes longer so doing it in the browser might be unfeasible for now. See https://github.com/anishathalye/neural-style

Anyway, even with Fast Neural Style, you can use any arbitrary image as a Style, but you'll have to train it first (4-6 hours on a GPU).