By whom? If you look at typical "image" hosters, you can't really tell outside of loading times if it is showing you a gif or a video. Are there browsers that do "clever" things with videos because they think there is audio?
That's a really big one for me. When I'm in 'silly consumption mode' I tend to just avoid anything that opens up as a video. I really wish there was an alternative to GIF for this use case though...
Instagram's Android app plays (MP4) videos [1] without audio by default. You just tap it to turn the audio on. I don't see why others couldn't do this?