I hate GIF and WebM so fucking much. Both of them, hate hate hate.
GIF is a great accessible format for general use, easy to capture your screen in, easy to animate stuff with, but it's a terrible format for the internet, especially if you're in charge of a website and in particular a content delivery network. GIF is big, takes a long time to load, looks fuckass terrible while it's interlacing, and can easily hobble a webpage if there's more than one of these fuckers onscreen at a time (don't even talk to me about GIF in iframes, holy shit).
In contrast, WebM is a great format for the internet, fixing everything that was wrong with GIF under the bonnet. They can load large, high-colour animations whip-cracking fast, taking up way less space both on disk and in the browser cache. But, it's a terrible format for general purpose. Most image editors can't figure out what the hell WebM is supposed to be and the ones that claim to know won't always load more than 1 frame. The only program that can open WebM is an internet browser, slowing file access way down because the browser has to load all of its own framework and add-ons, and then stream the image from your disk. This can be a privacy vulnerability if someone walks into the room because the browser loads and unloads so damn slowly and a security problem if you don't have the browser automatically clear the cache when you shut it off.
Loads of CDN's— tumblr included— will automatically convert GIF to WebM, but it's a damn nuisance trying to convert WebM back to GIF, since the average person can't do it without some external tool or a web app. I don't need to mention the inherent security vulnerability of loading your private images into a web app. "Oh, we won't look at your files," said the giant multinational corporation with its fingers crossed behind its back or the tech startup whose CEO has visions of fine Chianti and yachts in the French Riviera.
I hate GIF and WebM so fucking much. But not half as bad as GIFV! Whoever invented GIFV needs to get run over by trucks. Seriously, just use MP4 at that point, Jesus H. Christ.