WebP is Google's image format, designed in 2010 specifically for the web. It compresses 25-35% smaller than JPG at equivalent quality, supports transparency like PNG, and supports animation like GIF. Browser support is universal as of 2020 (Safari was the last holdout). The main reason most images aren't already WebP: legacy software (older Office versions, some email clients) doesn't open it.
How to open
All modern browsers display WebP natively. Photoshop added native support in version 23 (2021); earlier versions need a plugin. macOS Preview reads WebP since macOS Monterey. On Windows, the Photos app supports it; older viewers may not.