Safari doesn't support WebP, unless they added it recently, in which case there are still a lot of non-(WebP-supporting) Safari browsers out in the wild.
As always we need to wait for Apple to get their shirt together.
The <picture> tag provides a nice, no-JS <img> tag upgrade that works in browsers (it doesn't break in IE so long as you have an <img> in there) to deliver the right image to the user. That means the user gets a smaller download, faster to render, higher quality image. It's essentially progressive enhancement for images (everyone gets the JPG or PNG, some users get a better format if they can handle it).
You can even add media queries to the <source> tags to deliver different images based on the user's device.
caniuse.com currently states that "Safari is experimenting with supporting WebP images." I Googled around for more info, but the most conclusive thing I could find is this July 2016 blog post mentioning that "WebP Comes to iOS 10, macOS Sierra Betas". Since we're well past MacOS Sierra and iOS 10, does that mean Apple has given up trying to support WebP?
More realistically though until they support it I'll just stick to JPGs and PNGs and forget about this fallback mess. It adds unnecessary complexity to sites that are dynamically generated.
What this tells me is that when you build a site that has images uploaded by a user, you do nothing to those images. You just take the upload and stick it in an img tag to display it.
This is the exact problem with developers building for the Web. You'd rather avoid a relatively small amount of additional complexity than build something that's better for users (loads faster, reduces bandwidth, renders quicker).
Even ignoring WebP, you really should be resampling the upload to the right size and running it through something like imagemin to optimize the file size. And there are APIs out there that will do it all for you - it's a post request at most if you don't want to set it up on a server.
Not exactly true, this isn't about user-uploaded images. I get it, but optimized JPGs are working, and if it isn't broken, don't fix it. When everyone supports WebP I'll switch to WebP and ditch JPG but I'm not in any hurry and have plenty of other broken things that need to be fixed first.
As always we need to wait for Apple to get their shirt together.