Thank you for your reply.
I added your snippet to my .htaccess (root) but it doesn’t seem to work.
I uploaded a new image and added it to the test page https://www.vinicartasegna.it/prova/ . The WebP file il available https://www.vinicartasegna.it/wp-content/uploads/2023/05/virginia.jpeg.webp but it doesn’t be served .
If it helps, inside the wp-content/uploads directory there’s a .htaccess file with this code:
# BEGIN Register webp mime type
<IfModule mod_mime.c>
AddType image/webp .webp
</IfModule>
# END Register webp mime type
# BEGIN WP-Optimize WebP Rules
<IfModule mod_rewrite.c>
RewriteEngine On
# Redirect to existing converted image in same dir (if browser supports webp)
RewriteCond %{HTTP_ACCEPT} image/webp
RewriteCond %{REQUEST_FILENAME} (?i)(.*)(\.jpe?g|\.png)$
RewriteCond %1%2\.webp -f
RewriteRule (?i)(.*)(\.jpe?g|\.png)$ %1%2\.webp [T=image/webp,E=EXISTING:1,E=ADDVARY:1,L]
# Make sure that browsers which does not support webp also gets the Vary:Accept header
# when requesting images that would be redirected to webp on browsers that does.
<IfModule mod_headers.c>
<FilesMatch "(?i)\.(jpe?g|png)$">
Header append "Vary" "Accept"
</FilesMatch>
</IfModule>
</IfModule>
# Rules for handling requests for webp images
# ---------------------------------------------
# Set Vary:Accept header if we came here by way of our redirect, which set the ADDVARY environment variable
# The purpose is to make proxies and CDNs aware that the response varies with the Accept header
<IfModule mod_headers.c>
<IfModule mod_setenvif.c>
# Apache appends "REDIRECT_" in front of the environment variables defined in mod_rewrite, but LiteSpeed does not
# So, the next lines are for Apache, in order to set environment variables without "REDIRECT_"
SetEnvIf REDIRECT_EXISTING 1 EXISTING=1
SetEnvIf REDIRECT_ADDVARY 1 ADDVARY=1
Header append "Vary" "Accept" env=ADDVARY
# Set X-WPO-WebP header for diagnose purposes
Header set "X-WPO-WebP" "Redirected directly to existing webp" env=EXISTING
</IfModule>
</IfModule>
# END WP-Optimize WebP Rules