@skipper,
Really?
Yes, really... Maybe I wasn't clear, or maybe you misinterpreted the steps of my test and/or my remark.
To narrow size the problem area, I started without FeatherLight being installed, to only test the core of Grav. The fact that the webp image from the href (the one passed to featherlight) is correctly shown in the opened tab shows that the core of Grav is handling webp correctly.
Hence the conclusion that Grav is handling webp correctly.
After establishing that, I installed FeatherLight to test if webp is handled correctly by FeatherLight.
Just wanted to clarify that grav core does not support webp in a lightbox.
Grav core is handling webp correctly. The webp image is correctly resized, and the necessary html is also correctly generated.
It is the FeatherLight plugin that does not handle webp correctly.