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Regarding the technique itself, it is a nice hack, but it has its price. While disabling the speculative parser is essential to avoid downloading the same resource twice, it also means that the images will start to be downloaded only after the script ran.
That will have a performance cost, that in some extreme conditions (depending on the page and the user’s network) may exceed the cost of simply downloading the larger images…
It is unavoidable that the “same URL for different image dimensions” methods will mess up caching, while the “dynamically change the URL after the page loaded” methods will mess up the speculative parser.
Both these approaches have their performance costs. We should keep that in mind while waiting for a real solution from browser vendors and the W3C.