composer require felixhaeberle/kirby3-webp

git submodule add https://github.com/felixhaeberle/kirby3-webp.git site/plugins/kirby3-webp

return [
  'kirby3-webp' => true
]

<IfModule mod_rewrite.c>
  RewriteEngine On

  # Checking for WebP browser support ..
  RewriteCond %{HTTP_ACCEPT} image/webp

  # .. and if there's a WebP version for the requested image
  RewriteCond %{DOCUMENT_ROOT}/$1.webp -f

  # Well, then go for it & serve WebP instead
  RewriteRule (.+)\.(jpe?g|png)$ $1.webp [T=image/webp,E=accept:1]
</IfModule>

<IfModule mod_headers.c>
  Header append Vary Accept env=REDIRECT_accept
</IfModule>

<IfModule mod_mime.c>
  AddType image/webp .webp
</IfModule>

// First, make sure that NGINX' `mime.types` file includes 'image/webp webp'
include /etc/nginx/mime.types;

// Checking if HTTP's `ACCEPT` header contains 'webp'
map $http_accept $webp_suffix {
  default "";
  "~*webp" ".webp";
}

server {
  // ...

  // Checking if there's a WebP version for the requested image ..
  location ~* ^.+\.(jpe?g|png)$ {
    add_header Vary Accept;
    // .. and if so, serving it
    try_files $1$webp_ext $uri =404;
  }
}