How do mobile browsers render scaled images?

A responsive webpage is fit into the 320 pixel wide screen of a mobile device with the viewport tag. The page contains an image. The image file is 985 pixels wide, but scaled too 100% of its parent element through CSS, so in this mobile browser it displays at a width of 320 pixels.

When I zoom into the page, will I see the image at its original resolution (and in the original quality), or do mobile browsers recalculate scaled images and, when I zoom in, I see something similar to the pixelation of a zoomed thumbnail?