Well, this library is not SSR compatible, and there are only two reasons why not:

• README tells to add it to the head or bundle, while it has to be inlined just after body tag
• it uses onLoad/onError handlers which are async

Let's make it work right!

Long story short - here is working sandbox - https://codesandbox.io/s/crazy-ganguly-hmhe8

• it inlines base64 img before the body to tell the browser about it. The img is invisible.
• it "checks" img synchronously after creating, and it does work! (only with a real img tag, preload does not help).
• so it sets class to the body here and now, and the following HTML would use webp
• as long as dataURI inlined in the same document - it does not affect gzip size.

Tested

In Chrome(webp ✅), FireFox(webp ✅), and Safari(no webp, as expected)

Outcome

This library becomes SSR friendly!

Happy to open PR with a more compact version of the sandboxed example, however, the main changes are expected at the README level