In the developer notes, Apple notes that it has added WebP image support for the first time in Safari.
WebP is a newer image format developed by Google and announced in 2010. It provides lossy and lossless compression with smaller file sizes as compared to JPEG and PNG files. Compared with JPEGs, WebP is said to offer 25-35% smaller file sizes for the same quality. The addition of animation also allows it to also serve as a substitute to animated GIFs.
WebP images are supported in Chrome, Firefox and Microsoft Edge browsers, but has been notably absent from both iOS and macOS Safari. The addition to Safari makes the format more likely to receive widespread adoption.