If you care about the rendered quality, you should never use the browser itself to downscale images. Browsers don’t consistently use the state-of-the-art algorithm for downscaling, which is Lanczos 3-lobe. This is independent of partial IDCT. Browsers generally favor performance. And the algorithm chosen could even depend on the specific GPU in use.

That said if you are using the web to develop UI, it’s accepted that different browsers and users get slightly different renderings, unless you write your own image renderer with canvas. If you do care that much you should probably write native code.