WebP originally developed as a replacement for JPEG, PNG, and GIF file formats. It supports both lossy and lossless compression, animation and transparency. It has been actively developed since 2010, and is supported in all modern web browsers. Making new formats as common as todays mainstays necessitates them being accepted as one you can use for all purposes. Because of that, we believe WebP should be allowed as a source format.
The test is simply if each format is an accepted as source file for optimization. This is, by nature, custom to each tool and service.