enabling apache to serve WebP image files if accepted by browser (and .webp files available)

apache-webp-rewrite.conf

## !! This snippet has been updated, but not tested in practice. It should work, please check / leave comments in case it doesn't. !! ##

# originally from https://groups.google.com/a/webmproject.org/group/webp-discuss/browse_thread/thread/196ac4ea705688d8
<IfModule mod_rewrite.c>

  # TODO: don't forget to put
  # AddType image/webp .webp
  # in your mods-available/mime.conf
  # (though this is optional because we're setting
  # the mime type manually in the RewriteRule)

  # Enable rewrite 
  RewriteEngine On 

  # Does browser support WebP? 
  RewriteCond %{HTTP_ACCEPT} \bimage/webp\b 

  # Capture image name
  RewriteCond %{REQUEST_URI}  (.*)\.(jpe?g|png)$

  # if you don't have all jpg/png images available
  # as webp then you want to uncomment the next line
  # so apache first checks if there is a webp file
  # otherwise leave it disabled as it removes the
  # need to query the disk
  #RewriteCond %{DOCUMENT_ROOT}%1.webp -f

  # Route to WebP image 
  RewriteRule .* %1\.webp [L,T=image/webp]
  
  <IfModule mod_headers.c>
    <FilesMatch "\.(jpe?g|png)$">
      Header append Vary Accept
    </FilesMatch>
  </IfModule>
</IfModule>

Hi, sorry I didn't put any credits...

I was looking for this to enable webp on a single plain html page and landed here... I end up writing it... but tired of re-inventing the weal :-/

This is inspired by rules from wordpress plugin https://github.com/rosell-dk/webp-express (the guy has done an excellent job)

ADDVARY variable has a meaning and without it apache won't add the Vary header on a rewrite rule.

The other, I'm not sure... I know it works with it, but could have a meaning for apache as is could be for some other rules hook... in doubt I prefered keeping it.

I could not find documentation on ADDVARY magic variable, but I did test without it and Vary header goes missing too.
So when things magically work in what I consider as left overs from the 90s (fix one end brakes the other) I give up trying things.

This is based on heavily tested code... Wether one variable is needed over who knows what specific context, getting to that line took the dev of webp-express many many testing over many context... took him some effort.
I would just leave it untouched. As is it'll just work in the widest range of context encountered so far.

It ads the Vary header so it respects http standards...
This makes it compatible to use behind CDN...

Note: in case CDN ignored the Vary, one could add the 'Cache-Control: Private' which explicitly bans reverse proxy caching and still take advantage of browser cache.

Edit:
I did end up asking the question about the EXISTING variable... Best way to find out :)
rosell-dk/webp-express#380