Apple's Ricky Mondello has a great thread on some of the improvements for users and web developers coming to the next version of Safari, which is available in today's betas of iOS 11.3 and macOS 10.13.4.
Among the highlights: animated GIFs can be replaced with silent videos; Intelligent Tracking Prevention is getting even smarter; Safari Reader has an improved parser and support for link blogs; Password AutoFill for Apps, which debuted with iOS 11, now works in web views inside apps. If you're on the iOS 11.3 beta, you can try the improved Reader on this very post. As for GIFs, here at MacStories we already replaced them with silent .mp4 files loaded via native auto-play (see one in action here), but we're considering adding support for video content in
<img> tags as well. I'm also glad to see Apple expanding support for modern web app technologies including Service Workers and Web App Manifest.
You can find the full documentation for Safari 11.1 on Apple's website here.