WebP is a Google-developed image format supporting both lossy and lossless compression, with optional alpha channel and animation. Lossy WebP is typically 25-35% smaller than equivalent-quality JPEG; lossless WebP is typically 25% smaller than PNG. Browser support is universal as of 2023 (every major browser).
Use WebP for hero images, content images, and product photos -- anywhere you'd previously have used JPEG. Wrap in <picture> with a JPEG fallback for crawlers + email clients that still want JPEG: <picture><source type="image/webp" srcset="x.webp"><img src="x.jpg" alt="..."></picture>.
AVIF (a newer format) compresses even better (typically 50% smaller than JPEG) but encodes slower and lacks support in older browsers. The pragmatic 2026 stack: AVIF first, WebP second, JPEG fallback.