It should be noted that some sites (not sure about reddit) will re-encode images in various ways (sometimes to entirely different formats), possibly mitigating

It should be noted that some sites (not sure about reddit) will re-encode images in various ways (sometimes to entirely different formats), possibly mitigating this. Exploits aside, there are quite a few undesirable behaviours you can cause with media, such as the bouncing video file which changes it's height every frame and makes content around it reflow.