webm is an audiovisual media file format. It is primarily intended to offer a royalty-free alternative to use in the HTML5 video and the HTML5 audio elements. It has a sister project WebP for images. On Fedora system, you could use the Gnome’s embedded screencast tool to create a 30 seconds video of your screen by default. What I want is a gif animation that would be easier to be transferred and shown online.