Infinite looping zoom .gif generator in the browser

After a silly joke at work, I spent an evening implementing a Python script to generate "all the way down"-type animated gif images which endlessly zoom down onto some source image. I wanted this for a Slack emoji, but it'd be equally useful for Discord emoji and similar.

More specifically, design considerations were:

Avoid the pitfalls of other online generators which don't round properly. Depending whether the downscaled image had an odd or even number of pixels, the image would drift left and right through the animation.
Far too many .gif generators rely on server-side logic. I want to be able to play around with how my images are configured without incurring a bunch of network round trips, and without passing the images over to some third party service.
I wanted proper support for transparency. This was basically just to generate a "turtles all the way down" gif using the turtle emoji from Noto Color.
A desire to release it free to the world, for anyone to use (and modify).

The Python script met these needs at the time. However, I've had cases where I've wanted to generate these gifs but haven't been at a machine with the Pillow library installed. Thus a couple more evenings later...

All the way down in the browser

Using the Canvas API and the gif.js library

Please enjoy, and use the created gifs responsibly!