just had a friend with a Mac test it. when requesting foo.jpg (from an <img> tag), it sends:

  Accept: image/png, image/svg+xml, image/*;q=0, video/*;q=0.8, */*;q=0.5

the server responds with foo.jpg.

i also tested it in Firefox, which sends:

  Accept: image/webp, */*

the server responds with foo-converted.webp.

honestly, i'm not entirely sure how

  image/*;q=0
  */*

etc are handled, but it works; i also briefly looked at the plugin's source, and i haven't found any special handling for Safari. btw, here's a test page linked in their docs: http://toste.dk/rh.php

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edit: looking at the generated .htaccess, it appears that WebP is served only if Accept contains an explicit `image/webp` (no wildcards), so that explains it.