<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP_ACCEPT} image/webp
    RewriteRule (.+)\.png$ $1.png.webp [NC,T=image/webp,L]
</IfModule>
<IfModule mod_headers.c>
    Header always set Cache-Control "private"
    Header append Vary "Accept"
</IfModule>