As the stuff is rendered on the front-end how do you deal with tags where you do not even have the information to decide how they shall be parsed on the server?

This seems rather ignorant and, in my experience, leads to security issues, such as CVE-2023-38500 or CVE-2023-23627. This is not decidable on the server-side, so you will always mess stuff like this up. Sanitization can only work properly on the client for HTML.