formats: [format1, format2, (...)]
formats: [webp, original]
Array (yml sequence) of the image formats you'd like to generate, in decreasing order of preference. Browsers will render the first format they find and understand, so If you put jpg before webp, your webp images will never be used.
original does what you'd expect. To supply webp, you must have an imagemagick webp delegate installed. (Most package managers just name it ‘webp')
Supported formats are anything which imagemagick supports, and has an installed delegate. See a list by running
$ convert --version.