[LordDragonfang]

(Most) browsers actually start displaying an image before it's fully downloaded. In fact, many image formats/renderers are specifically designed with this property in mind, like jpeg which will render progressively less blurry versions of an image as the browser receives progressively higher-frequency components of the Fourier transform.

While the bytes are there temporarily, just like with all the other methods discussed, chrome at least eventually give up on downloading the "whole" image and displays a broken image sign in place of the Mona Lisa (and presumably prevents it from being cached and deletes what was there)

[pvillano]

it would be interesting if the download stopped after the second to last progressive layer but before the last byte; then the .crdownload renaming workaround wouldn't work.