I use a very basic setup of twentyseventeen and die most recent wp version. When I upload PNGs or JPGs they as a featured image, they are saved as a resized version on my uploads folder and used on the homepage and archive pages as a resized versions for the responsive images.
GIF files are also saved in the uploads folder but are not used as responsive image. Only the original size is referenced and loaded in HTML. The pixel dimension of my GIF is1920x890px:
Is there any reason for this behaviour? Why arn't the image sizes used if they exists as resized gif files generated by wordpress? I have no plugins installed. The gifs are not animated gifs.