IE10 is EOL on January 31 this month.
IE11 will be some time further.
Generally our baseline web standards support for Microsoft is what Microsoft has considered EOL.

However, that doesn't mean we can't mark webp safe within Composr and make it a choice of the user.

99% of the work was already done for this in v11. I just did the final thing of marking the format web-safe.

It wasn't actually on my radar that Firefox had implemented webp!

As for Composr, I don't really want us adopting webp for bundled images. It only is a very marginal improvement over jpeg for lossless and png for lossy, yet jpeg and png are very widely supported in graphics apps. I'd like to see one of the competing new lossy formats succeed and leapfrog webp. Google are pushing webp as they do like to push things they develop internally even if they are of marginal utility.

Yeah I am using png and jpg as the source, just converting to a more modern file type so I don't have to work with 2 file types in the templates. I have no intention of editing webp in graphics apps. I just read about all major modern browsers having support with the exception of Safari on iOS (which is a work in progress so will be supported soon) and I noticed I couldn't upload webp in Composr so thought I'd mention :) Also seeing a lot of .jiff images around the web these days, no idea what that's about. You can save them as jpg so I don't get the point and not much info on .jiff files.