Move the controls, watch the picture change, and take the Pillow code away with you.
Nothing is uploaded. The preview is drawn in your browser.
The preview is drawn with the browser's own canvas, so it is a close approximation rather than a pixel exact simulation of Pillow. The code is the real thing: run it and you will get what you see, give or take the small differences between two independent implementations of blur and resampling.
One difference is worth knowing. The browser blurs with a real Gaussian, and so does ImageFilter.GaussianBlur, but the radius means slightly different things in each, so a value that looks right here may need a small adjustment in Python.