Here's the test snippet that I quoted in this github comment (using disposable custom-sized images hosted on `dummyimage.com` which fortunately still works): https://github.com/webcompat/web-bugs/issues/12774#issuecomment-339094641 for what it's worth.
Bug 1473501 Comment 14 Edit History
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For what it's worth, here's the test snippet that I quoted in this github comment (using disposable custom-sized images hosted on `dummyimage.com` which fortunately still works): https://github.com/webcompat/web-bugs/issues/12774#issuecomment-339094641