Optimised images defined using srcset and parameters not used in tests

[pmeenan]

How are you detecting the browsers that support WebP? I'm going to go out on a limb and assume it is by UA-sniffing on the server for the HTML request. If so, your UA matching logic isn't dealing well with the PTST/xxxx that WebPageTest adds to the end of the browser UA string by default.

In the advanced settings you can "Preserve UA string" to use the unmodified UA string but I'd recommend to not use sniffing and use the picture element (responsive images) and let the browser pick: https://css-tricks.com/using-webp-images/