This is not a machine-learning upscaler. The image is drawn onto a canvas two or four times larger using the browser's high-quality smoothing, which blends neighbouring pixels to fill the new ones in between. A gentle 3 x 3 sharpening filter is then run over the result to counter some of the softness that enlarging always adds.
AI upscalers work differently: a trained model predicts plausible texture, such as hair strands or brick edges, that was never in the file. That can look impressive but also invents details. Interpolation never invents anything; what you see is a smoother, larger version of what was there.