It's now possible in Google Chrome Canary to indicate to website servers that you'd like (if possible) to get some webp images by sending a custom HTTP Request Accept Header 'image/webp'.

Instead of relying on a wonky User-Agent detection, this experiment (if promoted) will work all the time - even when proxy caches are involved.
You can already give it a try by enabling the eponymous experimental flag in chrome://flags and see how it goes. For information, webp is an alternative image format designed for the web. See

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