I tried to find that out — though these are conventional blurry shadows, not pixel art: codepen.io/y6nH/pen/YRmVvZ (click with shift or ctrl to add larger numbers).
That's amazing. I stopped after 1500ish, so I didn't find the limit, but that's truly incredible just how much the browser can handle. Although I did start to notice a slow down in how quickly it rendered starting around 800ish.
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