We are sorry to inform you that CKEditor 5 does not support Internet Explorer 11 yet.
Don't worry - we have it on our roadmap.Browser compatibility section
Complete solutions, ready to use
Unlike many other editors, CKEditor 5 is not just “collaboration ready”. CKEditor 5 and Letters come with everything you need to make your software a truly real-time collaborative application.
The collaboration server required for propagating real-time collaboration changes is available as a service (SaaS) and as a ready-to-use solution for installation on your servers (on-premise). The on-premise installation is a fully scalable solution that can run in your own data center or in your private cloud.