In reply to montyjohn:
I think generally, it's actually easier these days than it used to be tbh. We don't support Internet Explorer anymore and pretty much all the browsers use the same Webkit browser engine apart from Firefox. Safari usually do something weird now and again but this problem was due to 2 typos in my code :/
I didn't have any device old enough to test the fallback images before. The WebP images worked fine on my 2014 MacBook but it's up to date on software updates.
This site is great for seeing device support: https://caniuse.com/?search=webp